iu_client: sccp_sap_up: don't decode empty disconnect indications

Change-Id: I1fea8f19433d908583de71d8d2401bbb8c0c7900
diff --git a/src/iu_client.c b/src/iu_client.c
index 8da9a43..ce06f8a 100644
--- a/src/iu_client.c
+++ b/src/iu_client.c
@@ -799,7 +799,9 @@
 		if (!ue)
 			break;
 
-		rc = ranap_cn_rx_co(cn_ranap_handle_co, ue, msgb_l2(oph->msg), msgb_l2len(oph->msg));
+		rc = 0;
+		if (msgb_l2len(oph->msg) > 0)
+			rc = ranap_cn_rx_co(cn_ranap_handle_co, ue, msgb_l2(oph->msg), msgb_l2len(oph->msg));
 		break;
 	case OSMO_PRIM(OSMO_SCU_PRIM_N_DATA, PRIM_OP_INDICATION):
 		/* connection-oriented data received */