Ensure $current_screen global is set before applying admin_title filter

[westonruter]

Fixes #206.

[nuvoPoint]

We have implemented this fix and are keeping an eye on the log files for the next couple of hours to see if this fixes #206

[nuvoPoint]

We haven't see the fatal error yet in our logs, but now we are getting the following:

PHP Notice: Undefined index: hook_suffix in /var/www/html/wp-admin/includes/class-wp-screen.php on line 213

The only two things changed in our setup is this fix and #179

[westonruter]

@nuvoPoint Thanks for that. I've also set $hook_suffix to address this in ad7adab.

[nuvoPoint]

It seems to work fine now with $hook_suffix - thanks!