bsc: Initialize the subscr to NULL in the default case

The paging response should always have a TMSI or IMSI
and we should be able to find the subscriber using that. If
no IMSI/TMSI is present and we would still accept the LU
we would access the uninitialized memory.

openbsc/src/bsc/osmo_bsc_filter.c

@@ -83,6 +83,9 @@ static int handle_page_resp(struct gsm_subscriber_connection *conn, struct msgb
 	case GSM_MI_TYPE_IMSI:
 		subscr = subscr_active_by_imsi(conn->bts->network, mi_string);
 		break;
+	default:
+		subscr = NULL;
+		break;
 	}
 
 	if (!subscr) {