Fix a locking inversion in MixMonitor.

We need to unlock the audiohook before trying to lock
the channel, since the correct locking order is channel
then audiohook.
........

Merged revisions 421882 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@421883 65c4cc65-6c06-0410-ace0-fbb531ad65f3