Fix crash when a PC callback is reset

[jech]

• Move setting of PC state into separate function
• Avoid crash after a PC callback has been reset

Fixes #1871

[codecov]

Codecov Report

Merging #1872 (c282ff8) into master (d1839c7) will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1872      +/-   ##
==========================================
- Coverage   75.82%   75.68%   -0.15%     
==========================================
  Files          84       84              
  Lines        6048     6049       +1     
==========================================
- Hits         4586     4578       -8     
- Misses       1076     1084       +8     
- Partials      386      387       +1     

Flag	Coverage Δ
go	77.38% <100.00%> (-0.16%)	⬇️
wasm	71.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
peerconnection.go	75.13% <100.00%> (+0.19%)	⬆️
sctptransport.go	76.11% <0.00%> (-3.34%)	⬇️
dtlstransport.go	63.88% <0.00%> (-1.86%)	⬇️
datachannel.go	82.98% <0.00%> (-1.25%)	⬇️
icetransport.go	68.63% <0.00%> (+1.18%)	⬆️

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[Sean-Der]

Merged with ee255e8