BUG/MEDIUM: checks: unsubscribe for events on the old conn-stream on connect

When a new connection is established, if an old connection is still attached to
the current check, it must be detroyed. When it happens, the old conn-stream
must be used to unsubscribe for events, not the new one.

No backport is needed.
diff --git a/src/checks.c b/src/checks.c
index 15966c8..efc9ce2 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1660,7 +1660,8 @@
 	/* 3- release and replace the old one on success */
 	if (check->cs) {
 		if (check->wait_list.events)
-			cs->conn->mux->unsubscribe(cs, check->wait_list.events, &check->wait_list);
+			check->cs->conn->mux->unsubscribe(check->cs, check->wait_list.events,
+							  &check->wait_list);
 
 		/* We may have been scheduled to run, and the I/O handler
 		 * expects to have a cs, so remove the tasklet