Tweak stack limit values for gh20840.phpt

Graciously tested by Andy Postnikov (andypost).

See phpGH-21086
Closes phpGH-21275

Author: iluuu1994

ext/standard/tests/general_functions/gh20840.phpt

@@ -12,7 +12,7 @@ if (getenv('SKIP_ASAN')) {
 }
 ?>
 --INI--
-zend.max_allowed_stack_size=128K
+zend.max_allowed_stack_size=256K
 --FILE--
 <?php
 class Node {