Prevent a crash when a buddy gets offline.

(closes issue #16760)
Reported by: fiddur
Patches:
      248394.diff uploaded by fiddur (license 678)i with modifications by me
Tested by: fiddur, phsultan


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@253261 65c4cc65-6c06-0410-ace0-fbb531ad65f3