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/
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Merged revisions 427736 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 427737 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@427738 65c4cc65-6c06-0410-ace0-fbb531ad65f3