Description:
------------
When $smarty->debugging = true and using $smarty->assign 
to assign a template variable with the value of 
$errcontext, PHP segfaults when trying to output the 
debugging console for Smarty.

Reproduce code:
---------------
<?
        include('Smarty.class.php');
        $smarty = new Smarty();
        $smarty->debugging = true; // the error will not occur if debugging = false

        set_error_handler("error_handler",E_ALL);
        function error_handler($errno, $errstr, $errfile, $errline, $errcontext)
        {
                global $smarty;
                $smarty->assign('killme',$errcontext);

                $smarty->display('killsmarty.tpl'); // this file can be empty but it has to exist
                echo($errstr);
        }
        trigger_error("Uh-ho!",E_USER_ERROR);
?>


Expected result:
----------------
$errcontext should be assigned to Smarty template variable 
$killme.

Actual result:
--------------
#0  0xb7a56e40 in vfprintf () from /lib/tls/libc.so.6
#1  0xb7a73ffb in vsprintf () from /lib/tls/libc.so.6
#2  0xb7a5fe1e in sprintf () from /lib/tls/libc.so.6
#3  0x08277eb1 in _convert_to_string ()
#4  0x0827f684 in zend_make_printable_zval ()
#5  0x08277a1c in concat_function ()
#6  0x082a2b7e in execute ()
#7  0x0829d9c8 in execute ()
#8  0x0829bf83 in zend_get_user_opcode_handler ()
#9  0x00000000 in ?? ()