app_voicemail: leave_vociemail doesn't use last_message_index to store next message

app_voicemail: leave_vociemail doesn't use last_message_index to store next message

trivial change to bring inline with 1.6.2 1.8 and trunk.
The symptom was if msg0000 was missing, and the last was msg0004,
the next msgnum would be msg0000 when it should have been msg0005

(issue #18998)
Reported by: tootai
Patches:
      bug18998.diff2.txt uploaded by alecdavis (license 585)
Tested by: alecdavis

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@312290 65c4cc65-6c06-0410-ace0-fbb531ad65f3