Issue in ajax_get_json.php when $dkim_strength = 0

Reported by M Roji:

I found some bug, that make my script not working properly.

The bug is the code in file /usr/local/ispconfig/interface/web/mail/ajax_get_json.php

$dkim_strength = $app->functions->intval($mail_config['dkim_strength']);
if ($dkim_strength=='') $dkim_strength = 2048;

The right way is

$dkim_strength = $app->functions->intval($mail_config['dkim_strength']);
if ($dkim_strength==0) $dkim_strength = 2048;