Fix regression in Confbridge user count

When the restructuring work got committed to Confbridge in r375470 to
fix many open issues, it caused a regression in the reported count of
users when conference information was requested via CLI or manager.
This corrects the user count and user information displayed when
listing conference information from the CLI and manager.

(closes issue ASTERISK-20938)
Reported By: Timo Teras
Patches:
  confbridge-list.patch uploaded by Timo Teras (license 5409)
........

Merged revisions 379478 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379479 65c4cc65-6c06-0410-ace0-fbb531ad65f3