Bug 1194188: [MSE] Don't allow call to appendBuffer if media element is in error. r=cpearce

dom/media/mediasource/SourceBuffer.cpp

@@ -550,6 +550,15 @@ SourceBuffer::PrepareAppend(const uint8_t* aData, uint32_t aLength, ErrorResult&
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return nullptr;
   }
+
+  // If the HTMLMediaElement.error attribute is not null, then throw an
+  // InvalidStateError exception and abort these steps.
+  if (!mMediaSource->GetDecoder() ||
+      mMediaSource->GetDecoder()->IsEndedOrShutdown()) {
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return nullptr;
+  }
+
   if (mMediaSource->ReadyState() == MediaSourceReadyState::Ended) {
     mMediaSource->SetReadyState(MediaSourceReadyState::Open);
   }