Bug 1308922, ensure expected DOM tree operations when calling prepend, r=ehsan

dom/base/nsINode.cpp

@@ -1886,7 +1886,8 @@ nsINode::Prepend(const Sequence<OwningNodeOrString>& aNodes,
     return;
   }
 
-  InsertBefore(*node, mFirstChild, aRv);
+  nsCOMPtr<nsINode> refNode = mFirstChild;
+  InsertBefore(*node, refNode, aRv);
 }
 
 void