Ensure a NULL file while debugging cannot crash AEL.

(closes issue #17215)
 Reported by: vazir
 Patches: 
       20100518__issue17215.diff.txt uploaded by tilghman (license 14)
 Tested by: tilghman


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@272260 65c4cc65-6c06-0410-ace0-fbb531ad65f3