Bug 1441404, return early when appending null string to a text fragment, r=baku

--HG--
extra : rebase_source : 597d6424e3cfc1abdc667aebb311fee4425c8fde

dom/base/nsTextFragment.cpp

@@ -362,6 +362,10 @@ bool
 nsTextFragment::Append(const char16_t* aBuffer, uint32_t aLength,
                        bool aUpdateBidi, bool aForce2b)
 {
+  if (!aLength) {
+    return true;
+  }
+
   // This is a common case because some callsites create a textnode
   // with a value by creating the node and then calling AppendData.
   if (mState.mLength == 0) {