Fix memory leaks in app_voicemail when using IMAP storage or realtime config

This patch fixes two memory leaks:

1. When find_user is called with NULL as its first parameter, the voicemail
   user returned is allocated on the heap.  The inboxcount2 function uses
   find_user in such a fashion when counting new messages, and fails to free
   the resulting voicemail user object.

2. When populate_defaults is called on a voicemail user, it wipes whatever
   flags have been set on the object by copying over the global flags object.
   If the VM_ALLOCED flag was ste on the voicemail user prior to doing so,
   that flag is removed.  This leaks the voicemail user when free_user is later
   called.

(closes issue ASTERISK-19155)
Reported by: Filip Jenicek
patches:
  asterisk.patch2 uploaded by Filip Jenicek (license 6277)

Patch slightly modified for this commit.

Review: https://reviewboard.asterisk.org/r/2096
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Merged revisions 372268 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 372288 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 372289 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@372302 65c4cc65-6c06-0410-ace0-fbb531ad65f3