Don't crash when typing 'core set verbose' or 'core set debug' by themselves.

(closes issue #14219)
Reported by: jamesgolovich
Patches:
      asterisk-setverbosecrash.diff.txt uploaded by jamesgolovich (license 176)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168626 65c4cc65-6c06-0410-ace0-fbb531ad65f3