fix signal cannot be registered repeatedly

[matyhtf]

No description provided.

[codecov]

Codecov Report

Merging #4170 (498623b) into master (e86eac3) will increase coverage by 0.07%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #4170      +/-   ##
==========================================
+ Coverage   48.75%   48.82%   +0.07%     
==========================================
  Files          71       71              
  Lines       14252    14261       +9     
==========================================
+ Hits         6948     6963      +15     
+ Misses       7304     7298       -6     

Impacted Files	Coverage Δ
src/os/signal.cc	73.61% <72.72%> (+1.75%)	⬆️
src/server/worker.cc	48.94% <0.00%> (+0.26%)	⬆️
src/protocol/base.cc	63.15% <0.00%> (+1.97%)	⬆️
src/wrapper/event.cc	90.38% <0.00%> (+3.84%)	⬆️

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