Zend: fix zend_ast_size definition

It is better not to use sizeof(struct_with_flexible_array)
and instead rely on offsetof(type, member) like most
other similar wrappers do.

Author: crrodriguez

Zend/zend_ast.h

@@ -313,7 +313,8 @@ typedef void (*zend_ast_apply_func)(zend_ast **ast_ptr, void *context);
 ZEND_API void zend_ast_apply(zend_ast *ast, zend_ast_apply_func fn, void *context);
 
 static zend_always_inline size_t zend_ast_size(uint32_t children) {
-	return sizeof(zend_ast) - sizeof(zend_ast *) + sizeof(zend_ast *) * children;
+	ZEND_ASSERT(children > 0);
+	return XtOffsetOf(zend_ast, child) + (sizeof(zend_ast *) * children);
 }
 
 static zend_always_inline bool zend_ast_is_special(zend_ast *ast) {