When a data channel fails to be created, return nil instead of crashing.

BUG=webrtc:4619

Review-Url: https://codereview.webrtc.org/2860353002
Cr-Commit-Position: refs/heads/master@{#18034}

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

@@ -25,6 +25,9 @@
   rtc::scoped_refptr<webrtc::DataChannelInterface> dataChannel =
       self.nativePeerConnection->CreateDataChannel(labelString,
                                                    &nativeInit);
+  if (!dataChannel) {
+    return nil;
+  }
   return [[RTCDataChannel alloc] initWithNativeDataChannel:dataChannel];
 }