Prevent potential race condition in multiparty basic bridges.

For more details about the race condition see the linked review
at the bottom of this commit

(closes issue ASTERISK-21882)
Reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/2663



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@394232 65c4cc65-6c06-0410-ace0-fbb531ad65f3