app_confbridge: Fixed "CBAnn" channels not going away.

Fixed a ref leak in conf_handle_talker_cb() everytime the conference
bridge was found to report a channel's talker status change.  The
resulting leak caused the "CBAnn" channels and the conference bridge to
never be destroyed.

Thanks to Richard Kenner on the asterisk-user's list for locating the
problem.

Reported by: Richard Kenner
........

Merged revisions 413454 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@413455 65c4cc65-6c06-0410-ace0-fbb531ad65f3