app_voicemail: Fix unchecked bounds of myArray in IMAP_STORAGE.

In update_messages_by_imapuser(), messages were appended to a finite
array which resulted in a crash when an IMAP mailbox contained more
than 256 entries. This memory is now dynamically increased as needed.

Observe that this patch adds a bunch of XXX's to questionable code. See
the review (url below) for more information.

ASTERISK-24190 #close
Reported by: Nick Adams
Tested by: Nick Adams

Review: https://reviewboard.asterisk.org/r/4126/
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Merged revisions 426691 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 426692 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@426696 f38db490-d61c-443f-a65b-d21fe96a405b