3 issues:
1) RETURN_NULL() was used via the macro NODE_GET_OBJ(), but the function
returns false on failure and cannot return null according to its
stub.
2) The struct layout of the different implementors of libxml only
guarantees overlap between the node pointer and the document
reference, so accessing the std zend_object may not work.
3) DOC_GET_OBJ() wasn't using ZSTR_VAL().
Closes GH-16307.
