fix: correct range on arrays of interface objects

[mvertes]

Fixes #784.

[trois-six]

Hello,
I think that this breaks something in Traefik with Pilot:

$ go get github.com/containous/yaegi@9c51f6bb694888db4cebc2886680d41b98552105
go: github.com/containous/yaegi 9c51f6bb694888db4cebc2886680d41b98552105 => v0.8.14-0.20200723102504-9c51f6bb6948
go: downloading github.com/containous/yaegi v0.8.14-0.20200723102504-9c51f6bb6948
$ go build ./cmd/traefik
$ go run ./cmd/traefik/ --log.level=DEBUG --api.insecure --entrypoints.web.address=:8180   --experimental.pilot.token=xxx --experimental.plugins.cache.moduleName=github.com/containous/plugin-simplecache --experimental.plugins.cache.version=v0.1.1
INFO[0000] Configuration loaded from file: /home/me/src/softwares/go/src/github.com/containous/traefik/traefik.toml 
INFO[2020-07-23T13:03:02+02:00] Traefik version dev built on I don't remember exactly 
DEBU[2020-07-23T13:03:02+02:00] Static configuration loaded {"global":{"checkNewVersion":true},"serversTransport":{"maxIdleConnsPerHost":200},"entryPoints":{"traefik":{"address":":8080","transport":{"lifeCycle":{"graceTimeOut":10000000000},"respondingTimeouts":{"idleTimeout":180000000000}},"forwardedHeaders":{},"http":{}},"web":{"address":":10080","transport":{"lifeCycle":{"graceTimeOut":10000000000},"respondingTimeouts":{"idleTimeout":180000000000}},"forwardedHeaders":{},"http":{}}},"providers":{"providersThrottleDuration":2000000000,"file":{"watch":true,"filename":"../dynamic_conf.toml"}},"api":{"insecure":true,"dashboard":true},"log":{"level":"DEBUG","format":"common"},"accessLog":{"format":"common","filters":{},"fields":{"defaultMode":"keep","headers":{"defaultMode":"drop"}}},"experimental":{"pilot":{"token":"xxx"},"plugins":{"cache":{"moduleName":"github.com/containous/plugin-simplecache","version":"v0.1.1"}}}} 
INFO[2020-07-23T13:03:02+02:00] 
Stats collection is disabled.
Help us improve Traefik by turning this feature on :)
More details on: https://docs.traefik.io/contributing/data-collection/ 
DEBU[2020-07-23T13:03:02+02:00] loading of plugin: cache: github.com/containous/[email protected] 
2020/07/23 13:03:05 traefik.go:75: command traefik error: github.com/containous/plugin-simplecache: failed to import plugin code "github.com/containous/plugin-simplecache": runtime error: invalid memory address or nil pointer dereference
exit status 1
$
$
$ go get github.com/containous/[email protected]
$ go build ./cmd/traefik
$ go run ./cmd/traefik/ --log.level=DEBUG --api.insecure --entrypoints.web.address=:8180   --experimental.pilot.token=xxx --experimental.plugins.cache.moduleName=github.com/containous/plugin-simplecache --experimental.plugins.cache.version=v0.1.1
INFO[0000] Configuration loaded from file: /home/me/src/softwares/go/src/github.com/containous/traefik/traefik.toml 
INFO[2020-07-23T13:41:49+02:00] Traefik version dev built on I don't remember exactly 
DEBU[2020-07-23T13:41:49+02:00] Static configuration loaded {"global":{"checkNewVersion":true},"serversTransport":{"maxIdleConnsPerHost":200},"entryPoints":{"traefik":{"address":":8080","transport":{"lifeCycle":{"graceTimeOut":10000000000},"respondingTimeouts":{"idleTimeout":180000000000}},"forwardedHeaders":{},"http":{}},"web":{"address":":10080","transport":{"lifeCycle":{"graceTimeOut":10000000000},"respondingTimeouts":{"idleTimeout":180000000000}},"forwardedHeaders":{},"http":{}}},"providers":{"providersThrottleDuration":2000000000,"file":{"watch":true,"filename":"../dynamic_conf.toml"}},"api":{"insecure":true,"dashboard":true},"log":{"level":"DEBUG","format":"common"},"accessLog":{"format":"common","filters":{},"fields":{"defaultMode":"keep","headers":{"defaultMode":"drop"}}},"experimental":{"pilot":{"token":"xxx"},"plugins":{"cache":{"moduleName":"github.com/containous/plugin-simplecache","version":"v0.1.1"}}}} 
INFO[2020-07-23T13:41:49+02:00] 
Stats collection is disabled.
Help us improve Traefik by turning this feature on :)
More details on: https://docs.traefik.io/contributing/data-collection/ 
DEBU[2020-07-23T13:41:49+02:00] loading of plugin: cache: github.com/containous/[email protected] 
INFO[2020-07-23T13:41:53+02:00] Starting provider aggregator.ProviderAggregator {} 
DEBU[2020-07-23T13:41:53+02:00] Start TCP Server                              entryPointName=web
DEBU[2020-07-23T13:41:53+02:00] Start TCP Server                              entryPointName=traefik
INFO[2020-07-23T13:41:53+02:00] Starting provider *file.Provider {"watch":true,"filename":"../dynamic_conf.toml"} 
ERRO[2020-07-23T13:41:53+02:00] Cannot start the provider *file.Provider: field not found, node: plugins 
INFO[2020-07-23T13:41:53+02:00] Starting provider *traefik.Provider {}       
DEBU[2020-07-23T13:41:53+02:00] Configuration received from provider internal: {"http":{"routers":{"api":{"entryPoints":["traefik"],"service":"api@internal","rule":"PathPrefix(`/api`)","priority":2147483646},"dashboard":{"entryPoints":["traefik"],"middlewares":["dashboard_redirect@internal","dashboard_stripprefix@internal"],"service":"dashboard@internal","rule":"PathPrefix(`/`)","priority":2147483645}},"services":{"api":{},"dashboard":{},"noop":{}},"middlewares":{"dashboard_redirect":{"redirectRegex":{"regex":"^(http:\\/\\/(\\[[\\w:.]+\\]|[\\w\\._-]+)(:\\d+)?)\\/$","replacement":"${1}/dashboard/","permanent":true}},"dashboard_stripprefix":{"stripPrefix":{"prefixes":["/dashboard/","/dashboard"]}}}},"tcp":{},"tls":{}}  providerName=internal
DEBU[2020-07-23T13:41:53+02:00] Added outgoing tracing middleware api@internal  middlewareName=tracing middlewareType=TracingForwarder entryPointName=traefik routerName=api@internal
DEBU[2020-07-23T13:41:53+02:00] Added outgoing tracing middleware dashboard@internal  middlewareName=tracing middlewareType=TracingForwarder entryPointName=traefik routerName=dashboard@internal
DEBU[2020-07-23T13:41:53+02:00] Creating middleware                           entryPointName=traefik routerName=dashboard@internal middlewareName=dashboard_stripprefix@internal middlewareType=StripPrefix
DEBU[2020-07-23T13:41:53+02:00] Adding tracing to middleware                  entryPointName=traefik middlewareName=dashboard_stripprefix@internal routerName=dashboard@internal
DEBU[2020-07-23T13:41:53+02:00] Creating middleware                           entryPointName=traefik routerName=dashboard@internal middlewareType=RedirectRegex middlewareName=dashboard_redirect@internal
DEBU[2020-07-23T13:41:53+02:00] Setting up redirection from ^(http:\/\/(\[[\w:.]+\]|[\w\._-]+)(:\d+)?)\/$ to ${1}/dashboard/  middlewareName=dashboard_redirect@internal entryPointName=traefik routerName=dashboard@internal middlewareType=RedirectRegex
DEBU[2020-07-23T13:41:53+02:00] Adding tracing to middleware                  entryPointName=traefik routerName=dashboard@internal middlewareName=dashboard_redirect@internal
DEBU[2020-07-23T13:41:53+02:00] Creating middleware                           entryPointName=traefik middlewareName=traefik-internal-recovery middlewareType=Recovery
DEBU[2020-07-23T13:41:53+02:00] No default certificate, generating one       
^CINFO[2020-07-23T13:42:01+02:00] I have to go...                              
INFO[2020-07-23T13:42:01+02:00] Stopping server gracefully                   
DEBU[2020-07-23T13:42:01+02:00] Waiting 10s seconds before killing connections.  entryPointName=traefik
DEBU[2020-07-23T13:42:01+02:00] Waiting 10s seconds before killing connections.  entryPointName=web
ERRO[2020-07-23T13:42:01+02:00] accept tcp [::]:8080: use of closed network connection  entryPointName=traefik
ERRO[2020-07-23T13:42:01+02:00] accept tcp [::]:10080: use of closed network connection  entryPointName=web
ERRO[2020-07-23T13:42:01+02:00] close tcp [::]:8080: use of closed network connection  entryPointName=traefik
DEBU[2020-07-23T13:42:01+02:00] Entry point traefik closed                    entryPointName=traefik
ERRO[2020-07-23T13:42:01+02:00] close tcp [::]:10080: use of closed network connection  entryPointName=web
DEBU[2020-07-23T13:42:01+02:00] Entry point web closed                        entryPointName=web
INFO[2020-07-23T13:42:01+02:00] Server stopped                               
INFO[2020-07-23T13:42:01+02:00] Shutting down