Fix WaitExten(x,m(musicclass)) string termination.

The AST_CONTROL_HOLD MOH class from the WaitExten application can now be
queued onto a channel, passed over local channels with the /m option, and
passed over IAX channels.
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Merged revisions 367469 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 367470 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@367477 65c4cc65-6c06-0410-ace0-fbb531ad65f3