fixes attended transfer behavior when both transferee and transferer hung up

If both the transferer and transferee of a attended transfer hangup before
the new channel picks up, the new channel should be hung up as well as it
has no endpoint to talk to.  This mirrors the expected behavior used in 1.4. 

(closes issue #17444)
Reported by: corruptor



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@271831 65c4cc65-6c06-0410-ace0-fbb531ad65f3