If there was no named defined in a voicemail.conf mailbox

entry, then app_directory would crash when attempting to
read that entry from the file. We now check for the NULL
or empty string properly so that there will be no crash.

(closes issue #13804)
Reported by: bluecrow76



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@152646 65c4cc65-6c06-0410-ace0-fbb531ad65f3