USSD: Fix "ussd default-route"

Before this patch, the default route logic was not implemented.  The
user could specify a default-route, but it wouldn't be used by the
actual routing logic.  Let's fix that.

Change-Id: I0b04a75dc297f088f13da413d08c52e0747e46e6

src/hlr_ussd.c

@@ -507,6 +507,11 @@ int rx_proc_ss_req(struct osmo_gsup_conn *conn, const struct osmo_gsup_message *
 						ss->is_external = false;
 						ss->u.iuse = rt->u.iuse;
 					}
+				} else {
+					if (hlr->euse_default) {
+						ss->is_external = true;
+						ss->u.euse = hlr->euse_default;
+					}
 				}
 			}
 			/* dispatch unstructured SS to routing */