fix crash if label's type is STRING_TEXTURE and  label->sortAllChildren is called

If I call Label::sortAllChildren on a Label object manually, it will crash.
I found the reason is:
If Label is STRING_TEXTURE label type, a Sprite will add by Node::AddChild in Label::createSpriteWithFontDefinition function , so this Sprite will add without batch mode (_atlasIndex = INDEX_NOT_INITIALIZED)


test code:
--------------------
```c++
auto label = Label::create("test", "Marker Felt", 24);
this->addChild(label);
label->sortAllChildren();
```