sms_queue: Print dest msisdn instead of unknown subscriber

If subscriber is NULL, vlr_subscr_msisdn_or_name() returns string
"unknown", which is less informative than printing destination msisdn
expected for the queued sms.
This happens for instance if an sms was queued with Store&Forward and
destination subscriber is not currently registered

Change-Id: I4b8b54c9c41b17d4e1fa7ece63aa91a98036ef11
diff --git a/src/libmsc/sms_queue.c b/src/libmsc/sms_queue.c
index 274c712..afd878f 100644
--- a/src/libmsc/sms_queue.c
+++ b/src/libmsc/sms_queue.c
@@ -228,8 +228,10 @@
 		/* Is the subscriber attached? If not, go to next SMS */
 		if (!sms->receiver || !sms->receiver->lu_complete) {
 			LOGP(DLSMS, LOGL_DEBUG,
-			     "Subscriber %s is not attached, skipping SMS %llu\n",
-			     vlr_subscr_msisdn_or_name(sms->receiver), sms->id);
+			     "Subscriber %s%s is not attached, skipping SMS %llu\n",
+			     sms->receiver ? "" : "MSISDN-",
+			     sms->receiver ? vlr_subscr_msisdn_or_name(sms->receiver)
+					   : sms->dst.addr, sms->id);
 			sms_free(sms);
 			continue;
 		}