res_musiconhold.c: Release any format refs before memset().

* Clear the channel music_state pointer before destroying the music_state
object for safety.
........

Merged revisions 422037 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422038 65c4cc65-6c06-0410-ace0-fbb531ad65f3