AST-2012-011: Resolve heap corruption issue with voicemail

The heard and deleted arrays in the voicemail state structure were not
handled properly following the memory leak fix in r354890 and a fix for
an invalid free in r356797.  This could result in accessing and writing
into freed memory.  The allocation for these arrays has been reworked
to avoid the possibility of invalid frees, access of freed memory, and
crashes that were occurring as a result of this.

Locking around accesses and modifications of the voicemail state
structure members dh_arraysize, heard, and deleted has been added to
prevent simultaneous modification and access when IMAP storage is in
use.  If IMAP storage is not in use, this locking is not compiled in.

Review: https://reviewboard.asterisk.org/r/1994/
(closes issue ASTERISK-19923)
Reported by: Dan Delaney
Tested by: Dan Delaney, Julian Yap
Patches:
  vm_alloc_fix.diff uploaded by kmoore (license 6273)

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Merged revisions 369652 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369653 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369676 65c4cc65-6c06-0410-ace0-fbb531ad65f3