ObjC: Add null checks to HW encoder compressionOutputCallback

webrtc/sdk/objc/Framework/Classes/VideoToolbox/RTCVideoEncoderH264.mm

 /*
  *  Copyright (c) 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.
  *
  */
@@ skipping 130 matching lines @@
   return pixel_buffer;
 }
 
 // This is the callback function that VideoToolbox calls when encode is
 // complete. From inspection this happens on its own queue.
 void compressionOutputCallback(void *encoder,
                                void *params,
                                OSStatus status,
                                VTEncodeInfoFlags infoFlags,
                                CMSampleBufferRef sampleBuffer) {
+  RTC_CHECK(params);
   std::unique_ptr<RTCFrameEncodeParams> encodeParams(
       reinterpret_cast<RTCFrameEncodeParams *>(params));
+  RTC_CHECK(encodeParams->encoder);
   [encodeParams->encoder frameWasEncoded:status
                                    flags:infoFlags
                             sampleBuffer:sampleBuffer
                        codecSpecificInfo:encodeParams->codecSpecificInfo
                                    width:encodeParams->width
                                   height:encodeParams->height
                             renderTimeMs:encodeParams->render_time_ms
                                timestamp:encodeParams->timestamp
                                 rotation:encodeParams->rotation];
 }
@@ skipping 535 matching lines @@
   }
   _bitrateAdjuster->Update(frame.buffer.length);
 }
 
 - (RTCVideoEncoderQpThresholds *)scalingSettings {
   return [[RTCVideoEncoderQpThresholds alloc] initWithThresholdsLow:kLowH264QpThreshold
                                                                high:kHighH264QpThreshold];
 }
 
 @end