When sorting the stars after having formed a star, only do so for the active cells.

src/runner.c

@@ -1301,6 +1301,8 @@ void runner_do_all_stars_sort(struct runner *r, struct cell *c) {
   if (c->nodeID != engine_rank) error("Function called on a foreign cell!");
 #endif
 
+ if(!cell_is_active_stars(c, r->e) && !cell_is_active_hydro(c, r->e)) return;
+
   /* Shall we sort at this level? */
   if (c->hydro.super == c) {