AGI may be invoked from outside the dialplan

(closes issue #16510)
 Reported by: atis
 Patches: 
       20091223__issue16510.diff.txt uploaded by tilghman (license 14)
 Tested by: atis


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@236300 65c4cc65-6c06-0410-ace0-fbb531ad65f3