Make sure BaseChannel::rtp_transport_ is nullptr when not connected.

Make sure that BaseChannel::rtp_transport_ is set to nullptr if the
channel cannot be connected to a transport (RegisterRtpDemuxerSink()
fails).

Handle and report potential error case in RtpTransceiver if a transport
can't be connected to a channel (RegisterRtpDemuxerSink fails).

Also, clear cached header extensions synchronously when the worker and
network threads are configured to be one and the same.

Bug: chromium:472458706
Change-Id: Iaed062bbb86ebe5d714a4657c260be5db79d44f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/438740
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46545}