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.
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Merged revisions 398416 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 398417 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 398418 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398419 65c4cc65-6c06-0410-ace0-fbb531ad65f3