Add a missing unlock and properly handle the 'maxusers' setting on MeetMe

Add a missing unlock and properly handle the 'maxusers' setting on MeetMe
conferences.  We were using the 'user number' field to compare against the
maximum allowed users, which works assuming users with lower user numbers
didn't leave the conference.

(closes issue #14117)
Reported by: sergedevorop
Patches:
      20090114__bug14117-2.diff.txt uploaded by seanbright (license 71)
Tested by: sergedevorop

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@168705 65c4cc65-6c06-0410-ace0-fbb531ad65f3