nat: Remember if the MSC accepted the connection on principle

For the USSD module we only want to handle requests that are
legitimate. This is achieved by looking for the CM Service Accept
or the BSSAP CIPHER MODE COMMAND.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index 50c50ed..ff1d27a 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -318,6 +318,10 @@
 	if (!con->imsi)
 		return 0;
 
+	/* We have not verified the IMSI yet */
+	if (!con->authorized)
+		return 0;
+
 	if (!con->bsc->nat->ussd_lst_name)
 		return 0;
 	if (!con->bsc->nat->ussd_query)