ACP Logout redirect wrong $url if $dir is empty

In ACP
After click the "ACP Logout" and then redirected wth meta_refresh
the $redirect_url is something like
http://localhost/ ... /forum//index.php?sid=f54ed7...
which makes url problem in overall_header.php if using the

variable.

note : two slash before index.php

I found that it works ok if I made some changes inside
-> includes/function.php -> function redirect{ -> line 1905
$url = $dir . '/' . $url;
becomes:
if (!empty($dir))
{
$url = $dir . '/' . $url;
}

in my case the $dir variable is empty value, which causing the problem.