Fix: Cannot upload file to sub-folder in Media Manager

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33687&start=0
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33413&start=0

Fix based on (german):
http://www.joomlaportal.de/joomla-3-x-installation/309865-sammelthread-joomla-3-3-0-released-2.html#post1534645
http://www.joomla-bugs.de/forum/index.php?topic=582.0