Fix: Waitlist signup not working for full paid events

CiviCRM 4.4.x seems to have introduced a bug which does not allow participants to waitlist a full, paid event. 

There is a thread on the topic: http://forum.civicrm.org/index.php/topic,30689.msg131591.html 

The problem seems to come from the fact that the fee part of the form is still active, even when trying to waitlist. 

(This is  currently opened under: http://issues.civicrm.org/jira/browse/CRM-14225 and I previously submitted it as an issue on github, but the original issue URL now returns a 404)