bug: fix Conn.Next not safe (#648)

[zhongweikang]

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).

#648

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 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.93%. Comparing base (2e261de) to head (8e96be7).
Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
connection_unix.go	0.00%	3 Missing ⚠️
connection_windows.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #649      +/-   ##
==========================================
+ Coverage   78.57%   78.93%   +0.35%     
==========================================
  Files          21       21              
  Lines        2535     2535              
==========================================
+ Hits         1992     2001       +9     
+ Misses        417      412       -5     
+ Partials      126      122       -4     

Flag	Coverage Δ
unittests	78.93% <0.00%> (+0.35%)	⬆️

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[panjf2000]

Thanks!