Avoid close on closed channel

[aandryashin]

No description provided.

[codecov-io]

Codecov Report

Merging #318 into master will decrease coverage by 0.02%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage    74.2%   74.17%   -0.03%     
==========================================
  Files           9        9              
  Lines        1000     1003       +3     
==========================================
+ Hits          742      744       +2     
- Misses        211      212       +1     
  Partials       47       47

Flag	Coverage Δ
#unittests	74.17% <75%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
selenoid.go	77.18% <75%> (-0.09%)	⬇️

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