Bug 1344604 - part2: Return empty DocumentFragment if the ConvertCueToDOMTree() return null. r=alwu

It is a regression of bug1307710, if something wrong during the ConvertCueToDOMTree() in vtt.jsm, we will get null ptr.

MozReview-Commit-ID: LSQrJIhBzRU

--HG--
extra : rebase_source : b0a2cf675f9288ddc9c264faaffcf97049c6c1c6

dom/media/TextTrackCue.cpp

@@ -136,6 +136,9 @@ TextTrackCue::GetCueAsHTML()
   nsCOMPtr<nsIDOMDocumentFragment> frag;
   sParserWrapper->ConvertCueToDOMTree(window, this,
                                       getter_AddRefs(frag));
+  if (!frag) {
+    return mDocument->CreateDocumentFragment();
+  }
   return frag.forget().downcast<DocumentFragment>();
 }