app_mixmonitor: Fix crashes caused by unloading app_mixmonitor

Unloading app_mixmonitor while active mixmonitors were running would
cause a segfault. This patch fixes that by making it impossible to
unload app_mixmonitor while mixmonitors are active.

Review: https://reviewboard.asterisk.org/r/2624/
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Merged revisions 391778 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 391794 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391828 65c4cc65-6c06-0410-ace0-fbb531ad65f3