[#33720] Fix: Error in user registration with profile plugin enabled

## How to test
* enable the profile plugin
* register a new user
* use as bithday date: 99-99-2014
* apply patch
* test again
* now (the current date) save to the database.

Tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33720&start=0