tinywl: fix crash if there is no keyboard

To reproduce, run

WLR_BACKENDS=headles tinywl -s foot

tinywl/tinywl.c

@@ -114,7 +114,6 @@ 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);
@@ -125,6 +124,10 @@ static void focus_view(struct tinywl_view *view, struct wlr_surface *surface) {
 	 * track of this and automatically send key events to the appropriate
 	 * clients without additional work on your part.
 	 */
+	struct wlr_keyboard *keyboard = wlr_seat_get_keyboard(seat);
+	if (keyboard == NULL) {
+		return;
+	}
 	wlr_seat_keyboard_notify_enter(seat, view->xdg_surface->surface,
 		keyboard->keycodes, keyboard->num_keycodes, &keyboard->modifiers);
 }