Return nil from RTCPeerConnectionFactory when creation fails

RTCPeerConnectionFactory.createPeerConnection did not check the return
value of the native createPeerConnection() - so when the native PC
fails to be created, it could end up attempting to use a null pointer.

The change makes it return nil when the creation fails. The application
can then detect and respond to the failure.

Review-Url: https://codereview.webrtc.org/2240633004
Cr-Commit-Position: refs/heads/master@{#13732}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

@@ -232,6 +232,9 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
                                                     nullptr,
                                                     nullptr,
                                                     _observer.get());
+    if (!_peerConnection) {
+      return nil;
+    }
     _localStreams = [[NSMutableArray alloc] init];
     _delegate = delegate;
   }