Avoid nullptr dereference in RTCPeerConnectionHandler

Bug: 1071327
Change-Id: Icf4189905dc5c95854b5af4b3e5e25e0607dd39e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153325Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759775}

third_party/blink/renderer/modules/peerconnection/rtc_peer_connection_handler.cc

@@ -2384,7 +2384,8 @@ void RTCPeerConnectionHandler::OnRemoveReceiverPlanB(uintptr_t receiver_id) {
 
 void RTCPeerConnectionHandler::OnModifySctpTransport(
     blink::WebRTCSctpTransportSnapshot state) {
-  client_->DidModifySctpTransport(state);
+  if (client_)
+    client_->DidModifySctpTransport(state);
 }
 
 void RTCPeerConnectionHandler::OnModifyTransceivers(
@@ -2519,7 +2520,8 @@ void RTCPeerConnectionHandler::OnIceCandidateError(
 }
 
 void RTCPeerConnectionHandler::OnInterestingUsage(int usage_pattern) {
-  client_->DidNoteInterestingUsage(usage_pattern);
+  if (client_)
+    client_->DidNoteInterestingUsage(usage_pattern);
 }
 
 webrtc::SessionDescriptionInterface*