[skip ci] Reduce zend.max_allowed_stack_size in gh20840.phpt

This test still fails on i386 Windows with "Allowed memory size of %d bytes
exhausted" because the output buffer grows too big. My first intuition was to
add a chunk_size to ob_start, but this won't work if the output buffer is
flushed deep into the call stack, causing a premature or just a second stack
limit error. So, reduce the stack size in an attempt to produce less output.

ext/standard/tests/general_functions/gh20840.phpt

@@ -12,7 +12,7 @@ if (getenv('SKIP_ASAN')) {
 }
 ?>
 --INI--
-zend.max_allowed_stack_size=512K
+zend.max_allowed_stack_size=128K
 --FILE--
 <?php
 class Node {