Provide a little more protection to make sure that a MOH class is specified...

 Provide a little more protection to make sure that a MOH class is specified when using the 'm' option with WaitExten, and prevent a segfault in the process.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@41930 65c4cc65-6c06-0410-ace0-fbb531ad65f3