Improvements on checks in admin/index.php

I fixed a small issue that gave the error `Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 24576 bytes) in /path/to/admin/index.php on line 78` if user tries to access `http://domain/admin/index.php`.

Also I added an else statement to output an error to the user if no page has been passed to `admin/index.php`.