fix problem when using get.php (some constants are not defined)

If you call a request to a file in /media/ folder, it will be rewritten and handled by get.php
Unfortunately get.php doesn't define constants like mage.php does.
so you'll get error like:
Notice: Use of undefined constant BP - assumed 'BP' in app/code/local/Varien/Autoload.php on line 64

This patch solves this problem.