ui/update_model: do not recalculate the max_idx If there is no lead (#23356)

selfdrive/ui/ui.cc

@@ -71,11 +71,11 @@ static void update_model(UIState *s, const cereal::ModelDataV2::Reader &model) {
   auto model_position = model.getPosition();
   float max_distance = std::clamp(model_position.getX()[TRAJECTORY_SIZE - 1],
                                   MIN_DRAW_DISTANCE, MAX_DRAW_DISTANCE);
+  int max_idx = get_path_length_idx(model_position, max_distance);
 
   // update lane lines
   const auto lane_lines = model.getLaneLines();
   const auto lane_line_probs = model.getLaneLineProbs();
-  int max_idx = get_path_length_idx(lane_lines[0], max_distance);
   for (int i = 0; i < std::size(scene.lane_line_vertices); i++) {
     scene.lane_line_probs[i] = lane_line_probs[i];
     update_line_data(s, lane_lines[i], 0.025 * scene.lane_line_probs[i], 0, &scene.lane_line_vertices[i], max_idx);
@@ -94,8 +94,8 @@ static void update_model(UIState *s, const cereal::ModelDataV2::Reader &model) {
   if (lead_one.getStatus()) {
     const float lead_d = lead_one.getDRel() * 2.;
     max_distance = std::clamp((float)(lead_d - fmin(lead_d * 0.35, 10.)), 0.0f, max_distance);
-  }
     max_idx = get_path_length_idx(model_position, max_distance);  
+  }
   update_line_data(s, model_position, 0.5, 1.22, &scene.track_vertices, max_idx);
 }