Avoid race when stopping audio unit on iOS

webrtc/modules/audio_device/ios/audio_device_ios.mm

 /*
  *  Copyright (c) 2012 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 804 matching lines @@
 
   // Release the lock.
   [session unlockForConfiguration];
 
   return true;
 }
 
 void AudioDeviceIOS::ShutdownPlayOrRecord() {
   LOGI() << "ShutdownPlayOrRecord";
 
+  // Stop the audio unit to prevent any additional audio callbacks.
+  audio_unit_->Stop();
+
   // Close and delete the voice-processing I/O unit.
   audio_unit_.reset();
 
   // Remove audio session notification observers.
   RTCAudioSession* session = [RTCAudioSession sharedInstance];
   [session removeDelegate:audio_session_observer_];
 
   // All I/O should be stopped or paused prior to deactivating the audio
   // session, hence we deactivate as last action.
   [session lockForConfiguration];
   UnconfigureAudioSession();
   [session endWebRTCSession:nil];
   [session unlockForConfiguration];
 }
 
 }  // namespace webrtc