[ticket/11842] Use group_id 0 and correct avatar name after creating group

It seems like the function group_correct_avatar() was forgotten while adding
the new avatar system. We now pass the group_id 0 to the upload avatar and
let the function group_correct_avatar() fix the avatar filename after creating
the group like it was done previously.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11842

PHPBB3-11842