Fix leak in AMI Action Bridge

Add missing reference cleanup for newly created bridge.

ASTERISK-24281
Reported by: Stefan Engström
Review: https://reviewboard.asterisk.org/r/4154/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427736 f38db490-d61c-443f-a65b-d21fe96a405b