Fixed error about Undefined property JTableUser::$checked_out when saving a user

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31324

When saving a user, and php error reporting is on max, you'll get an error about Undefined property JTableUser::$checked_out.
This PR fixes the issue.

#### Testing Instructions
Save a user and see if the error is no longer there.
Also check saving other stuff, like articles (test with items that are checked out and checked in).