message: Fix for defect found by clusterfuzz.
  Cause: VideoRtpReceiver::media_channel_ was used when it was null.
  Fix: only use when provably not null.
  Cr-Commit-Position: refs/heads/master@{#30017}
diff: @@ -155,6 +155,7 @@ void VideoRtpReceiver::RestartMediaChannel(absl::optional<uint32_t> ssrc) {
  }
  void VideoRtpReceiver::SetSink(rtc::VideoSinkInterface<VideoFrame>* sink) {
  + RTC_DCHECK(media_channel_);
  if (ssrc_) {
  media_channel_->SetSink(*ssrc_, sink);
  return;
  @@ -271,6 +272,11 @@ std::vector<RtpSource> VideoRtpReceiver::GetSources() const {
  void VideoRtpReceiver::OnGenerateKeyFrame() {
  RTC_DCHECK_RUN_ON(worker_thread_);
  + if (!media_channel_) {
  + RTC_LOG(LS_ERROR)
  + << "VideoRtpReceiver::OnGenerateKeyFrame: No video channel exists.";
  + return;
  + }
  // TODO(bugs.webrtc.org/8694): Stop using 0 to mean unsignalled SSRC
  media_channel_->GenerateKeyFrame(ssrc_.value_or(0));
  // We need to remember to request generation of a new key frame if the media
  