Discard link to media channel when audio sender stopped.

Bug: chromium:822799
Change-Id: Ib863cf048318b04369cc51ed1b1c8b03010a2fd2
Reviewed-on: https://webrtc-review.googlesource.com/62941
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22503}

pc/rtpsender.cc

@@ -250,6 +250,7 @@ void AudioRtpSender::Stop() {
       stats_->RemoveLocalAudioTrack(track_.get(), ssrc_);
     }
   }
+  media_channel_ = nullptr;
   stopped_ = true;
 }
 
@@ -433,6 +434,7 @@ void VideoRtpSender::Stop() {
   if (can_send_track()) {
     ClearVideoSend();
   }
+  media_channel_ = nullptr;
   stopped_ = true;
 }