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

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/

This patch is a remedial cherry-pick from v13.

Change-Id: I73fdf13a3a1042566281c7d06d6e83e2ef87c120