bug: fix Conn.Next and Conn.Peek panic On Unix

[serious-snow]

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

bug-fixes

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

The code was modified at the suggestion of the author。

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

#615

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

nothing

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

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.43%. Comparing base (5346527) to head (6269155).

Files	Patch %	Lines
connection_unix.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #616      +/-   ##
==========================================
- Coverage   79.58%   79.43%   -0.15%     
==========================================
  Files          21       21              
  Lines        2106     2106              
==========================================
- Hits         1676     1673       -3     
- Misses        298      300       +2     
- Partials      132      133       +1     

Flag	Coverage Δ
unittests	79.43% <25.00%> (-0.15%)	⬇️

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[panjf2000]

Thanks!