Avoid race when stopping audio unit on iOS

webrtc/modules/audio_device/ios/audio_device_ios.mm

Index: webrtc/modules/audio_device/ios/audio_device_ios.mm
diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.mm b/webrtc/modules/audio_device/ios/audio_device_ios.mm
index 0bb997db53c425ec03cf84811cef7891063e870f..1eddd208e8f4fe484d8c921f50bc34e35ab7d8c0 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.mm
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.mm
@@ -822,6 +822,9 @@ bool AudioDeviceIOS::InitPlayOrRecord() {
 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();