Bridge core: Pass a ref with the swap channel when joining a bridge.

When code imparts a channel into a bridge to swap with another channel, a
ref needs to be held on the swap channel to ensure that it cannot
dissapear before finding it in the bridge.

* The ast_bridge_join() swap channel parameter now always steals a ref for
the swap channel.  This is the only change to the bridge framework's
public API semantics.

* bridge_channel_internal_join() now requires the bridge_channel->swap
channel to pass in a ref.

ASTERISK-24649
Reported by: John Bigelow

Review: https://reviewboard.asterisk.org/r/4354/
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Merged revisions 430975 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430976 65c4cc65-6c06-0410-ace0-fbb531ad65f3