opcache: use zend_ast_size helper in zend_persist_ast

Author: crrodriguez

ext/opcache/zend_persist.c

@@ -188,7 +188,7 @@ static zend_ast *zend_persist_ast(zend_ast *ast)
 		node = (zend_ast *) copy;
 	} else {
 		uint32_t children = zend_ast_get_num_children(ast);
-		node = zend_shared_memdup(ast, sizeof(zend_ast) - sizeof(zend_ast *) + sizeof(zend_ast *) * children);
+		node = zend_shared_memdup(ast, zend_ast_size(children));
 		for (i = 0; i < children; i++) {
 			if (node->child[i]) {
 				node->child[i] = zend_persist_ast(node->child[i]);