Fix greeting retrieval from IMAP

Properly check for the current voicemail state and if it doesn't exist,
create it.

(closes issue #14597)
Reported by: wtca
Patches:
      14597_v2.patch uploaded by mmichelson (license 60)
Tested by: jpeeler



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@213404 65c4cc65-6c06-0410-ace0-fbb531ad65f3