chan_iax2: Fix stray reference to worker thread idle_list.

* Fix stray reference to idle_list in cleanup_thread_list().  This may be
the reason for the note in iax2_process_thread() about threads not being
removed from the task lists.

* Move cleanup_thread_list(&idle_list) to after the other lists are
cleaned up.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@398416 f38db490-d61c-443f-a65b-d21fe96a405b