Fix how we skip fields (to avoid fields which don't exist) when doing an UPDATE.

(closes issue #14205)
 Reported by: maxgo
 Patches: 
       20090128__bug14205__5.diff.txt uploaded by Corydon76 (license 14)
 Tested by: blitzrage


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@172131 65c4cc65-6c06-0410-ace0-fbb531ad65f3