Fix a voicemail memory leak with heard/deleted messages.

open_mailbox() was changed quite a long time ago to allocate this memory.
close_mailbox() should have been changed to be responsible for freeing it.
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Merged revisions 354889 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 354890 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@354903 65c4cc65-6c06-0410-ace0-fbb531ad65f3