Update handling of asyncgoto so it properly works on channels that are currently executing a PBX.

(closes issue #11914)
Reported by: arnd
(closes issue #11753)
Reported by: johan


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@102272 65c4cc65-6c06-0410-ace0-fbb531ad65f3