Let vm_mailbox_snapshot combine "Urgent" when no folder is specified

r381835 fixed a bug in vm_mailbox_snapshot where combining INBOX and Old forgot
that Urgent also "counts" as new messages. This fixed the problem when any of
the three folders was specified and the combine option was used.

It missed the case where the folder isn't specified and we build a snapshot of
all folders. This patch corrects that.
........

Merged revisions 382617 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@382621 65c4cc65-6c06-0410-ace0-fbb531ad65f3