BUG/MEDIUM: connections: Don't forget to detach the connection from the SI.

When we're deferring the mux choice until the ALPN is negociated, we
attach the connection to the stream_interface until it's done, so that we
can destroy it if something goes wrong and the stream is destroy.
Before calling si_attach_cs() to attach the conn_stream once we have it,
call si_detach_endpoint(), or is_attach_cs() would destroy the connection.

src/backend.c

@@ -1056,6 +1056,7 @@ static int conn_complete_server(struct connection *conn)
 		goto fail;
 	if (conn->flags & CO_FL_ERROR)
 		goto fail;
+	si_detach_endpoint(&s->si[1]);
 	cs = si_alloc_cs(&s->si[1], conn);
 	if (!cs)
 		goto fail;