Fixed observer unsubscribtion in RTCRtpReceiver.

Missing unsubscribtion caused accessing invalid pointer inside
AudioRtpReceiver::OnFirstPacketReceived on short-lived
RTCRtpReceiver objects.

Bug: webrtc:6112
Change-Id: I5df141628e1cfd69aff59177d395c3246e1bf367
Reviewed-on: https://webrtc-review.googlesource.com/54306
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22157}

sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm

@@ -72,6 +72,12 @@ void RtpReceiverDelegateAdapter::OnFirstPacketReceived(
       self.receiverId];
 }
 
+- (void)dealloc {
+  if (_nativeRtpReceiver) {
+    _nativeRtpReceiver->SetObserver(nullptr);
+  }
+}
+
 - (BOOL)isEqual:(id)object {
   if (self == object) {
     return YES;