trivial fix for an agi error when attempting to use EAGI on a dead/hungup...

trivial fix for an agi error when attempting to use EAGI on a dead/hungup channel, we now print an error that makes sense 
given our removal of deadagi as an actual application.

(closes issue #12161)
Reported by: explidous
Patches:
      res_agi_12161.patch uploaded by juggie (license 24)
Tested by: juggie


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@106399 65c4cc65-6c06-0410-ace0-fbb531ad65f3