fix(baseTrack): panic on close of nil channel

[adityaa30]

panic: close of nil channel

goroutine 281 [running]:
github.com/pion/mediadevices.(*baseTrack).bind.func1.1()
	/Users/naruto/go/pkg/mod/github.com/pion/mediadevices@v0.3.12/track.go:190 +0x54
github.com/pion/mediadevices.(*baseTrack).bind.func1()
	/Users/naruto/go/pkg/mod/github.com/pion/mediadevices@v0.3.12/track.go:204 +0x120
created by github.com/pion/mediadevices.(*baseTrack).bind
	/Users/naruto/go/pkg/mod/github.com/pion/mediadevices@v0.3.12/track.go:186 +0x30c


Process finished with the exit code 1

[codecov]

Codecov Report

Base: 58.08% // Head: 58.05% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (8e0c671) compared to base (76ba048).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
- Coverage   58.08%   58.05%   -0.04%     
==========================================
  Files          62       62              
  Lines        3691     3693       +2     
==========================================
  Hits         2144     2144              
- Misses       1420     1422       +2     
  Partials      127      127              

Impacted Files	Coverage Δ
track.go	24.53% <0.00%> (-0.16%)	⬇️

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[edaniels]

I'm not seeing where the channel is ever not initialized or explicitly set to nil. Can you point me at that or provide a reproduction of the panic you mention?