Bugfix: correctly check for GTP initialization

The existing check for the initialization of GTP is wrong; GTP returns
-1 if it could not initialize. Correct this, such that the DU will stop
if GTP cannot be initialize.

If we continue, the DU is requested to set up a GTP tunnel; because GTP
is not initialized, it assumes it is in monolithic/no F1, and does not
set up a tunnel. The F1 UE Context setup response encoder then fails
with this assertion:

    Assertion (tnl->teid > 0) failed!
    In DU_send_UE_CONTEXT_SETUP_RESPONSE() ../../../openair2/F1AP/f1ap_du_ue_context_management.c:415

    Exiting execution
    ../../../openair2/F1AP/f1ap_du_ue_context_management.c:415 DU_send_UE_CONTEXT_SETUP_RESPONSE() Exiting OAI softmodem: _Assert_Exit_
    Aborted

Closes: #853

openair2/F1AP/f1ap_du_task.c

@@ -142,7 +142,7 @@ void *F1AP_DU_task(void *arg) {
         createF1inst(myInstance, msgSetup, nc);
         du_task_send_sctp_association_req(myInstance, nc);
         instance_t gtpInst = du_create_gtpu_instance_to_cu(nc);
-        AssertFatal(gtpInst != 0, "cannot create DU F1-U GTP module\n");
+        AssertFatal(gtpInst > 0, "cannot create DU F1-U GTP module\n");
         getCxt(myInstance)->gtpInst = gtpInst;
         DUuniqInstance = gtpInst;
       } break;