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: http://svn.asterisk.org/svn/asterisk/branches/11@410965 f38db490-d61c-443f-a65b-d21fe96a405b