app_queue: Initialize array holding MixMonitor exec options

If the channel variable MONITOR_EXEC is set, app_queue will pass the specified
execution parameters to the MixMonitor application when a queue is recorded.
If that channel variable is not set, the buffer that holds the escaped value
was not being initialized to NULL, and so would be passed to the MixMonitor
application with garbage. Hilarity ensued as app_mixmonitor attempted to
execute gobeldy-gook.
........

Merged revisions 399681 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399682 65c4cc65-6c06-0410-ace0-fbb531ad65f3