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.
........

Merged revisions 398416 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@398417 65c4cc65-6c06-0410-ace0-fbb531ad65f3