Fix for preventing close of stopRead nil channel

[malykhin]

Description

The behaviour is described here: #457
The issue is opened for Windows, but it occurs on Raspbian as well. Can't reproduce it on macOS.
Noticed this issue after migrating mediadevices from v0.3.7 to v0.3.10

Reference issue

Fixes #457

[codecov]

Codecov Report

Base: 58.15% // Head: 58.16% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (d6925fd) compared to base (cb23f1f).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   58.15%   58.16%   +0.01%     
==========================================
  Files          62       62              
  Lines        3680     3679       -1     
==========================================
  Hits         2140     2140              
+ Misses       1413     1412       -1     
  Partials      127      127              

Impacted Files	Coverage Δ
track.go	25.31% <0.00%> (+0.07%)	⬆️

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[wdouglass]

This looks like a positive change, Thanks! Would you also move the creation of stopRead from line 229 up above line 185? that way it won't be null in the first place...