bridge.c: Hangup attended transfer target after it has been swapped out

After completing an attended transfer the transfer target channel (the one that
gets swapped out) was not being hung up after leaving the bridge. This resulted
in a channel possibly being left around. Added an explicit softhangup for the
channel in question after the transfer is successfully completed in order to
make sure the channel is hung up.

ASTERISK-24782 #close
Reported by: John Bigelow
Review: https://reviewboard.asterisk.org/r/4575/
........

Merged revisions 434240 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434241 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/bridge.c

@@ -4538,6 +4538,7 @@ enum ast_transfer_result ast_bridge_transfer_attended(struct ast_channel *to_tra
 		ast_bridge_unlock(to_transferee_bridge);
 		ast_bridge_unlock(to_target_bridge);
 
+		ast_softhangup(to_transfer_target, AST_SOFTHANGUP_DEV);
 		goto end;
 	}