Fix opus codec panic on read after close

[at-wat]

Also add test to other codecs

[codecov[bot]]

Codecov Report

Merging #440 (ec4a647) into master (8ad810e) will increase coverage by 0.19%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #440      +/-   ##
==========================================
+ Coverage   47.61%   47.81%   +0.19%     
==========================================
  Files          67       67              
  Lines        4469     4476       +7     
==========================================
+ Hits         2128     2140      +12     
+ Misses       2214     2209       -5     
  Partials      127      127              

Impacted Files	Coverage Δ
pkg/codec/opus/opus.go	67.74% <100.00%> (+2.62%)	:arrow_up:
pkg/codec/vpx/vpx.go	83.51% <0.00%> (+0.53%)	:arrow_up:
track.go	25.23% <0.00%> (+0.63%)	:arrow_up:
pkg/codec/x264/x264.go	65.11% <0.00%> (+1.16%)	:arrow_up:
pkg/codec/openh264/openh264.go	81.15% <0.00%> (+1.44%)	:arrow_up:

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