sms db: don't attempt to query pending SMS for unset MSISDN

When the subscriber has no MSISDN, we might construct an invalid SQL statement
such as

  ... AND dest_addr= AND ...

Instead, don't even query for empty MSISDNs.

Related: OS#2706
Change-Id: I7d6169d774b2da04b3051957e364fe620feed51e

src/libmsc/db.c

@@ -840,6 +840,10 @@ struct gsm_sms *db_sms_get_unsent_for_subscr(struct vlr_subscr *vsub,
 	if (!vsub->lu_complete)
 		return NULL;
 
+	/* A subscriber having no phone number cannot possibly receive SMS. */
+	if (*vsub->msisdn == '\0')
+		return NULL;
+
 	result = dbi_conn_queryf(conn,
 		"SELECT * FROM SMS"
 		" WHERE sent IS NULL"