avfilter/palettegen: misc cosmetics

libavfilter/vf_palettegen.c

@@ -188,7 +188,7 @@ static int get_next_box_id_to_split(PaletteGenContext *s)
         return -1;
 
     for (box_id = 0; box_id < s->nb_boxes; box_id++) {
-        struct range_box *box = &s->boxes[box_id];
+        const struct range_box *box = &s->boxes[box_id];
         if (s->boxes[box_id].len >= 2 && box->cut_score > max_score) {
             best_box_id = box_id;
             max_score = box->cut_score;