bug: fix the last error being missed in ReleaseTimeout() for multi-pool

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.77%. Comparing base (b40e489) to head (03cee03).

Files	Patch %	Lines
multipool.go	66.66%	0 Missing and 1 partial ⚠️
multipool_func.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #334      +/-   ##
==========================================
- Coverage   90.90%   90.77%   -0.14%     
==========================================
  Files          11       11              
  Lines         836      824      -12     
==========================================
- Hits          760      748      -12     
  Misses         51       51              
  Partials       25       25              

Flag	Coverage Δ
unittests	90.77% <66.66%> (-0.14%)	⬇️

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