Bug 1294648: [MSE] Return early if mediasource isn't attached. r=gerald

MozReview-Commit-ID: 9hCi5sPbZpW

--HG--
extra : rebase_source : 23b4cd16d3e104a30c0f1709b9ce0d04a105d219

dom/media/mediasource/MediaSourceDecoder.cpp

@@ -114,6 +114,10 @@ MediaSourceDecoder::GetBuffered()
 {
   MOZ_ASSERT(NS_IsMainThread());
 
+  if (!mMediaSource) {
+    NS_WARNING("MediaSource element isn't attached");
+    return media::TimeIntervals::Invalid();
+  }
   dom::SourceBufferList* sourceBuffers = mMediaSource->ActiveSourceBuffers();
   if (!sourceBuffers) {
     // Media source object is shutting down.