Repair IAXVAR implementation so that it works again (regression?)

(closes issue #13354)
 Reported by: adomjan
 Patches: 
       20080828__bug13354.diff.txt uploaded by Corydon76 (license 14)
       20080829__bug13354__1.6.0.diff.txt uploaded by Corydon76 (license 14)
 Tested by: Corydon76, adomjan


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@143031 65c4cc65-6c06-0410-ace0-fbb531ad65f3