Janitor patch for chan_misdn (make channel variable access safe)

(closes issue #12887)
 Reported by: pputman
 Patches: 
       chan_misdn_threadsafe.patch uploaded by pputman (license 81)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168591 65c4cc65-6c06-0410-ace0-fbb531ad65f3