app_confbridge: Make explicitly stop MOH if a user is kicked or hangs up while MOH is playing.

When MOH is playing to a user in a conference and the user is kicked or
hangs up from the conference then the AMI MusicOnHoldStop events didn't
happen.  (Asterisk v11 AMI event: MusicOnHold, state:Stop)

(closes issue ASTERISK-23311)
Reported by: Benjamin Keith Ford

Review: https://reviewboard.asterisk.org/r/3306/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@410490 f38db490-d61c-443f-a65b-d21fe96a405b