When playing back a prompt to a channel, confbridge neglects to check for
hangup events causing lockup condititions for hangups that occur before
actually joining the conference. This change ensures that the user is removed
from the conference in the event of a premature hangup.
Review: https://reviewboard.asterisk.org/r/1277/
git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@324305 65c4cc65-6c06-0410-ace0-fbb531ad65f3