Mailbox list would previously grow at each reload, containing duplicates.

Also, optimize the allocation of mailboxes to avoid additional memory structures.

(closes issue #16320)
 Reported by: Marquis
 Patches: 
       20100525__issue16320.diff.txt uploaded by tilghman (license 14)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@268817 65c4cc65-6c06-0410-ace0-fbb531ad65f3