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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@391778 f38db490-d61c-443f-a65b-d21fe96a405b