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

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

 /*
  *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import "RTCPeerConnection+Private.h"
 
 #import "NSString+StdString.h"
 #import "RTCDataChannel+Private.h"
 #import "RTCDataChannelConfiguration+Private.h"
 
 @implementation RTCPeerConnection (DataChannel)
 
 - (RTCDataChannel *)dataChannelForLabel:(NSString *)label
                           configuration:
     (RTCDataChannelConfiguration *)configuration {
   std::string labelString = [NSString stdStringForString:label];
   const webrtc::DataChannelInit nativeInit =
       configuration.nativeDataChannelInit;
   rtc::scoped_refptr<webrtc::DataChannelInterface> dataChannel =
       self.nativePeerConnection->CreateDataChannel(labelString,
                                                    &nativeInit);
+  if (!dataChannel) {
+    return nil;
+  }
   return [[RTCDataChannel alloc] initWithNativeDataChannel:dataChannel];
 }
 
 @end