Fix multi-station answer race condition.

When an SLA trunk is ringing (inbound call on the trunk) Asterisk will
make outbound calls to the stations that have that trunk.  If more than
one station answers the call at the same time, all channels other than
the first one to answer are left in a bad state.  The channel gets
leaked, is not connected to anything, and there's no way to get rid of
it.

We now properly clean up these losing channels by hanging up on them.
Since they lost the race, as we process their answer, there is no
ringing trunk for them to answer.
........

Merged revisions 383835 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@383836 65c4cc65-6c06-0410-ace0-fbb531ad65f3