Make the guardtime work even if the date is < the year 1970, and if something...

Make the guardtime work even if the date is < the year 1970, and if something weird happens in zap, don't stack overflow


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@3636 65c4cc65-6c06-0410-ace0-fbb531ad65f3