[topology-hiding] fix detection of the UAC requests

Use a more generix approach (check VIA2 presence) to see if we deal with a locally generated or proxyed requests.
Fixes #2690, credits go to @bcnewlin for reporting.

modules/topology_hiding/topo_hiding_logic.c

@@ -1036,11 +1036,10 @@ static void topo_dlg_onroute (struct dlg_cell* dlg, int type,
 		return;
 	}
 
-	/* we also may end up here via TERMINATE event triggered by internal
-	 * dlg termination -> the requests we have here are dummy, so nothing
-	 * to be done */
-	if (is_dummy_sip_msg(req)==0) {
-		LM_DBG("dummy request identified, skipping...\n");
+	/* filter out the case we end up here for local UAC request (one VIA), as
+	 * there is no relaying to follow, so nothing to be done here */
+	if (req->via2==NULL) {
+		LM_DBG("uac/dummy request identified, skipping...\n");
 		return;
 	}