Description:
------------
When asked to encode a circular data structure, json_encode does not notice.

Reproduce code:
---------------
<?php
$data = array('a' => 12);
$data['bad'] =& $data;
var_dump(json_encode($data));
?>

Expected result:
----------------
Some sort of error about object recursion, as this is not supported by JSON.

Actual result:
--------------
PHP Fatal error:  Allowed memory size of 25165824 bytes exhausted (tried to allocate 128 bytes) ...