`bufReadConn` be closed twice in `closeConn`

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

https://github.com/pingcap/tidb/pull/49104/files#diff-412615e1c1d4c6ae11a7885a11c879778bf401e0967c9c13c4b9dfd1f13ed29aR357-R371
I think in above PR, there may be some mistakes, it is quite different with the original PR.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

v6.5.8

I think it happens on release-6.5 and release-7.1.