Bug 1129877 - Fix the crash. v1 r=mattwoodrow

We could also fix this by invoking .Reject() on the holder instead of on the raw
promise. But there's actually no reason to involve the holder at all here.

dom/media/fmp4/MP4Reader.cpp

@@ -556,10 +556,7 @@ MP4Reader::RequestVideoData(bool aSkipToNextKeyframe,
 
   if (mShutdown) {
     NS_WARNING("RequestVideoData on shutdown MP4Reader!");
-    MonitorAutoLock lock(mVideo.mMonitor);
-    nsRefPtr<VideoDataPromise> p = mVideo.mPromise.Ensure(__func__);
-    p->Reject(CANCELED, __func__);
-    return p;
+    return VideoDataPromise::CreateAndReject(CANCELED, __func__);
   }
 
   MOZ_ASSERT(HasVideo() && mPlatform && mVideo.mDecoder);
@@ -590,14 +587,14 @@ MP4Reader::RequestAudioData()
 {
   MOZ_ASSERT(GetTaskQueue()->IsCurrentThreadIn());
   VLOG("RequestAudioData");
+  if (mShutdown) {
+    NS_WARNING("RequestAudioData on shutdown MP4Reader!");
+    return AudioDataPromise::CreateAndReject(CANCELED, __func__);
+  }
+
   MonitorAutoLock lock(mAudio.mMonitor);
   nsRefPtr<AudioDataPromise> p = mAudio.mPromise.Ensure(__func__);
-  if (!mShutdown) {
-    ScheduleUpdate(kAudio);
-  } else {
-    NS_WARNING("RequestAudioData on shutdown MP4Reader!");
-    p->Reject(CANCELED, __func__);
-  }
+  ScheduleUpdate(kAudio);
   return p;
 }