Fix bridging thread leak.

The bridge thread was exiting but was never being
reaped using pthread_join(). This has been fixed now
by calling pthread_join() in ast_bridge_destroy().

(closes issue ASTERISK-19834)
Reported by Marcus Hunger

Review: https://reviewboard.asterisk.org/r/2012
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Merged revisions 369708 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 369709 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369710 65c4cc65-6c06-0410-ace0-fbb531ad65f3