Fix issue of seg. fault at xnjsmb_coord_invalidate_subtree(). It caused by _xnjsmb_coord_clone_from_subtree_mod() does not create an JS wrapper object every descendants of the subtree. We do not provide any method for traveling tree. So, it is fine to clone a subtree without creating wrapper objects for descendants. So, we check if an object own a JS wrapper object before invalidating it.