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

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

Index: webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+DataChannel.mm
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+DataChannel.mm b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+DataChannel.mm
index b3825d4e4ebb55ae39bd1dc10853fcb63d683b09..80b510898c078b64de53117b7d216032f08c08f8 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+DataChannel.mm
+++ b/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];
 }