Router: fixed segmentation fault.

In the case that routes or upstreams is empty and the pass option is a variable.
If the resolved pass is routes or upstreams, a segment error occurred.

src/nxt_http_route.c

@@ -1567,6 +1567,10 @@ nxt_http_route_find(nxt_http_routes_t *routes, nxt_str_t *name,
 {
     nxt_http_route_t  **route, **end;
 
+    if (routes == NULL) {
+        return NXT_DECLINED;
+    }
+
     route = &routes->route[0];
     end = route + routes->items;