Fix deadlock potential with ast_set_hangupsource() calls.

Calling ast_set_hangupsource() with the channel lock held can result in a
deadlock because the function also locks the bridged channel.

(issue ASTERISK-19537)

(closes issue AST-891)
Reported by: Guenther Kelleter
Tested by: Guenther Kelleter

(closes issue ASTERISK-19801)
Reported by: Alec Davis
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Merged revisions 368759 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 368760 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@368772 65c4cc65-6c06-0410-ace0-fbb531ad65f3