Add NULL check for DU lookup in handover notify

Add early return if get_du_by_cell_id returns NULL to prevent
segmentation fault when accessing DU structure members.

openair2/RRC/NR/rrc_gNB_NGAP.c

@@ -1457,11 +1457,15 @@ void rrc_gNB_send_NGAP_HANDOVER_REQUEST_ACKNOWLEDGE(gNB_RRC_INST *rrc, gNB_RRC_U
 void rrc_gNB_send_NGAP_HANDOVER_NOTIFY(gNB_RRC_INST *rrc, gNB_RRC_UE_t *UE)
 {
   LOG_I(NR_RRC, "Triggering NGAP Handover Notify\n");
+  nr_rrc_du_container_t *du = get_du_by_cell_id(rrc, rrc->nr_cellid);
+  if (du == NULL) {
+    LOG_E(NR_RRC, "Failed to send Handover Notify: no DU found with NR Cell ID=%lu\n", rrc->nr_cellid);
+    return;
+  }
 
   MessageDef *msg_p = itti_alloc_new_message(TASK_RRC_GNB, 0, NGAP_HANDOVER_NOTIFY);
   ngap_handover_notify_t *ho_notify = &NGAP_HANDOVER_NOTIFY(msg_p);
   memset(ho_notify, 0, sizeof(*ho_notify));
-  nr_rrc_du_container_t *du = get_du_by_cell_id(rrc, rrc->nr_cellid);
 
   ho_notify->gNB_ue_ngap_id = UE->rrc_ue_id;
   ho_notify->amf_ue_ngap_id = UE->amf_ue_ngap_id;