Fix channel reference leak in ChanSpy.

* Fix next_channel() channel reference leak in ChanSpy.

(closes issue ASTERISK-19461)
Reported by: Irontec
Patches:
      app_chanspy_iteartor_next_unref.patch (license #6213) patch uploaded by Irontec

(issue ASTERISK-17515)
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Merged revisions 357809 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 357810 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@357815 65c4cc65-6c06-0410-ace0-fbb531ad65f3