Fix crash when configuration does not match hardware detection.

(closes issue #12096)
 Reported by: mmickan
 Patches: 
       chan_vpb.cc.diff uploaded by mmickan (license 400)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@104974 65c4cc65-6c06-0410-ace0-fbb531ad65f3