Bug 1493347: Drop DataChannelListener on Destroy() r=drno

netwerk/sctp/datachannel/DataChannel.cpp

@@ -376,6 +376,7 @@ DataChannelConnection::Destroy()
 
   MOZ_ASSERT(mSTS);
   ASSERT_WEBRTC(NS_IsMainThread());
+  mListener = nullptr;
   // Finish Destroy on STS thread to avoid bug 876167 - once that's fixed,
   // the usrsctp_close() calls can move back here (and just proxy the
   // disconnect_all())