Description:
------------
Applying isset() to a dereference element of the string causes a very small leak.

Reproduce code:
---------------
<?php
$a = "string";
var_dump(isset($a{0}));
?>


Expected result:
----------------
bool(true)

Actual result:
--------------
/home/koizumi/src/php-src-4/Zend/zend_execute.c(2399) : Freeing 0x0837545C (2 bytes), script=/tmp/test.php

 2397:     zval *str = T->EA.data.str_offset.str;
 2398:     char c = str->value.str.val[T->EA.data.str_offset.offset];
 2399:     T->tmp_var.value.str.val = estrndup(&c, 1);
 2400:     T->tmp_var.value.str.len = 1;
 2401:     isset = 1;
 2402: }