Remove pointless ZVAL_UNDEF() in isset path (php#18187)

This will already be set to UNDEF by zend_call_function.

Author: ndossche

Zend/zend_object_handlers.c

@@ -877,7 +877,6 @@ ZEND_API zval *zend_std_read_property(zend_object *zobj, zend_string *name, int
 
 		if (!((*guard) & IN_ISSET)) {
 			GC_ADDREF(zobj);
-			ZVAL_UNDEF(&tmp_result);
 
 			*guard |= IN_ISSET;
 			zend_std_call_issetter(zobj, name, &tmp_result);