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: http://svn.asterisk.org/svn/asterisk/trunk@394232 f38db490-d61c-443f-a65b-d21fe96a405b