nat: Fix a crash due the refactoring and pass the nat again.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index ff1d27a..af12340 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -251,7 +251,7 @@
 
 	nat->ussd_listen.data = nat;
 	return make_sock(&nat->ussd_listen, IPPROTO_TCP,
-			 ntohl(addr.s_addr), 5001, 0, ussd_listen_cb, NULL);
+			 ntohl(addr.s_addr), 5001, 0, ussd_listen_cb, nat);
 }
 
 static int forward_ussd(struct sccp_connections *con, const struct ussd_request *req,