app_confbridge: Fix bug - users with startmuted set don't start muted

(closes issue ASTERISK-23461)
Reported by: Chico Manobela
Review: https://reviewboard.asterisk.org/r/3373/


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@410965 65c4cc65-6c06-0410-ace0-fbb531ad65f3