Allow double close of codecs

[at-wat]

Video/AudioTrack.NewRTPReader() internally closes encoder on error.
It caused double free if user closes reader after error.

[codecov]

Codecov Report

Merging #364 (b84cced) into master (1f313a9) will increase coverage by 0.11%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   46.30%   46.41%   +0.11%     
==========================================
  Files          67       67              
  Lines        4354     4369      +15     
==========================================
+ Hits         2016     2028      +12     
- Misses       2214     2218       +4     
+ Partials      124      123       -1     

Impacted Files	Coverage Δ
pkg/codec/vaapi/vp8.go	0.00% <0.00%> (ø)
pkg/codec/vaapi/vp9.go	0.00% <0.00%> (ø)
pkg/codec/openh264/openh264.go	72.41% <100.00%> (+1.50%)	⬆️
pkg/codec/opus/opus.go	62.79% <100.00%> (+1.34%)	⬆️
pkg/codec/vpx/vpx.go	81.91% <100.00%> (+0.29%)	⬆️
pkg/codec/x264/x264.go	64.70% <0.00%> (+3.52%)	⬆️

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