fix: check stdConn buffer is nil when ResetBuffer

[hihaha]

---

name: Pull request
about: fix stdConn ResetBuffer function on windows
title: ''
labels: ''
assignees: ''

1. Are you opening this pull request for bug-fixs, optimizations or new feature?

2. Please describe how these code changes achieve your intention.

3. Please link to the relevant issues (if any).

4. Which documentation changes (if any) need to be made/updated because of this PR?

4. Checklist

• I have squashed all insignificant commits.
• I have commented my code for explaining package types, values, functions, and non-obvious lines.
• I have written unit tests and verified that all tests passes (if needed).
• I have documented feature info on the README (only when this PR is adding a new feature).
• (optional) I am willing to help maintain this change if there are issues with it later.

[codecov]

Codecov Report

Merging #122 into master will decrease coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   86.00%   85.59%   -0.41%     
==========================================
  Files          18       18              
  Lines        1400     1402       +2     
==========================================
- Hits         1204     1200       -4     
- Misses        161      166       +5     
- Partials       35       36       +1     

Impacted Files	Coverage Δ
connection_windows.go	77.39% <100.00%> (+0.40%)	⬆️
eventloop_unix.go	70.55% <0.00%> (-3.69%)	⬇️

---

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[panjf2000]

I don't think that buffer would be empty before the reset in ResetBuffer(), did you experience this kind of issue?

[hihaha]

I don't think that buffer would be empty before the reset in ResetBuffer(), did you experience this kind of issue?

是的，遇到过这个问题，当前PR修改不完善，后面会重新提交一个PR