Fix ORTC accept datachannel sctpTransport is nil

[jerry-tao]

Description

When using ORTC API, the new accepted channel don't set the sctpTransport, see https://github.com/pion/webrtc/blob/master/sctptransport.go#L219.

it will panic if someone try to use it:

pair, err := channel.Transport().Transport().ICETransport().GetSelectedCandidatePair()

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.12 ⚠️

Comparison is base (570b61a) 77.75% compared to head (b069de7) 77.64%.

❗ Current head b069de7 differs from pull request most recent head be7a893. Consider uploading reports for the commit be7a893 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2303      +/-   ##
==========================================
- Coverage   77.75%   77.64%   -0.12%     
==========================================
  Files          87       87              
  Lines        9318     9276      -42     
==========================================
- Hits         7245     7202      -43     
+ Misses       1646     1644       -2     
- Partials      427      430       +3     

Flag	Coverage Δ
go	79.42% <100.00%> (-0.12%)	⬇️
wasm	70.15% <ø> (ø)

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

Impacted Files	Coverage Δ
datachannel.go	85.12% <100.00%> (-0.82%)	⬇️
peerconnection.go	78.29% <100.00%> (-0.28%)	⬇️
sctptransport.go	77.90% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[Sean-Der]

Sorry I missed this @jerry-tao ! I will merge right now, thank you so much for the contribution :)