Assert that ApplicationData has epoch != 0

[Sean-Der]

Otherwise we may accept unencrypted/unauthenticated ApplicationData
from a remote

[codecov-io]

Codecov Report

Merging #128 into master will decrease coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
- Coverage   72.68%   72.51%   -0.18%     
==========================================
  Files          54       54              
  Lines        3141     3143       +2     
==========================================
- Hits         2283     2279       -4     
- Misses        606      609       +3     
- Partials      252      255       +3

Impacted Files	Coverage Δ
conn.go	78.18% <0%> (-0.41%)	⬇️
internal/udp/conn.go	78.99% <0%> (-3.37%)	⬇️

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