Bug 1361944. P2 - be sure to shut down the existing decoder before loading the next source child. r=cpearce

Otherwise we will fail the assertion (!mDecoder, "Shouldn't have a decoder")
in HTMLMediaElement::InitializeDecoderForChannel().

MozReview-Commit-ID: 1zShp0th3Uv

dom/html/HTMLMediaElement.cpp

@@ -5249,6 +5249,10 @@ void HTMLMediaElement::DecodeError(const MediaResult& aError)
   AudioTracks()->EmptyTracks();
   VideoTracks()->EmptyTracks();
   if (mIsLoadingFromSourceChildren) {
+    if (mDecoder) {
+      // Shut down the exiting decoder before loading the next source child.
+      ShutdownDecoder();
+    }
     mErrorSink->ResetError();
     if (mSourceLoadCandidate) {
       DispatchAsyncSourceError(mSourceLoadCandidate);