app_queue: Cleanup ao2_iterator

Clean ao2_iterator, resolving reference leak to queue members.

ASTERISK-24454 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4111/
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Merged revisions 426255 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 426260 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 426266 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@426272 65c4cc65-6c06-0410-ace0-fbb531ad65f3