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/
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Merged revisions 410965 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 410966 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410967 65c4cc65-6c06-0410-ace0-fbb531ad65f3