Ignore Redial softkey when no previous dialed number is known

(closes issue #17126)
Reported by: wedhorn
Patches: 
      skinny79xx_redial1.diff uploaded by wedhorn (license 30)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@255851 65c4cc65-6c06-0410-ace0-fbb531ad65f3