webrtc: fix crash during client disconnection (#1482)

[aler9]

Fixes #1482

OnConnectionStateChange of pion/webrtc is not thread safe. Add a mutex to make it thread safe.

[codecov-commenter]

Codecov Report

Merging #1501 (108c9d2) into main (8ffdd9c) will increase coverage by 0.18%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1501      +/-   ##
==========================================
+ Coverage   65.45%   65.63%   +0.18%     
==========================================
  Files         118      118              
  Lines       12984    12988       +4     
==========================================
+ Hits         8499     8525      +26     
+ Misses       3814     3788      -26     
- Partials      671      675       +4     

Impacted Files	Coverage Δ
internal/core/webrtc_conn.go	54.85% <100.00%> (-0.04%)	⬇️
internal/core/source_static.go	75.00% <0.00%> (-4.17%)	⬇️
internal/core/path.go	72.05% <0.00%> (+2.10%)	⬆️
internal/hls/muxer_variant_fmp4_playlist.go	66.26% <0.00%> (+4.86%)	⬆️

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