app_voicemail: close_mailbox needs to respect additional messages while mailbox is...

app_voicemail: close_mailbox needs to respect additional messages while mailbox is open.

close_mailbox leave gaps in message sequence if messages are deleted and new messages
arrive during this time, this is because the shuffle down to slot 0, only shuffles
the number of pre-existing messages when mailbox is opened, ignoring new arrivals.

Fix: in close_mailbox re-evaluate number of messages before the shuffle, this then includes new arrivals.

Happens on filebased or ODBC storage.

(issues #19032,#18582,#18692,#18998)
Reported by: alecdavis,tootai,afosorio

Review: https://reviewboard.asterisk.org/r/1153/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@312070 65c4cc65-6c06-0410-ace0-fbb531ad65f3