tinywl: fix crash if there is no keyboard

To reproduce, run

WLR_BACKENDS=headles tinywl -s foot

tinywl/tinywl.c

@@ -94,11 +94,12 @@ struct tinywl_keyboard {
 
 static void focus_view(struct tinywl_view *view, struct wlr_surface *surface) {
 	/* Note: this function only deals with keyboard focus. */
-	if (view == NULL) {
-		return;
-	}
 	struct tinywl_server *server = view->server;
 	struct wlr_seat *seat = server->seat;
+	struct wlr_keyboard *keyboard = wlr_seat_get_keyboard(seat);
+	if (view == NULL || keyboard == NULL) {
+		return;
+	}
 	struct wlr_surface *prev_surface = seat->keyboard_state.focused_surface;
 	if (prev_surface == surface) {
 		/* Don't re-focus an already focused surface. */
@@ -114,7 +115,6 @@ static void focus_view(struct tinywl_view *view, struct wlr_surface *surface) {
 					seat->keyboard_state.focused_surface);
 		wlr_xdg_toplevel_set_activated(previous, false);
 	}
-	struct wlr_keyboard *keyboard = wlr_seat_get_keyboard(seat);
 	/* Move the view to the front */
 	wl_list_remove(&view->link);
 	wl_list_insert(&server->views, &view->link);