Update the stars' ti_old_part when receiving stars, not the gravity one.

src/runner.c

@@ -3016,7 +3016,7 @@ void runner_do_recv_spart(struct runner *r, struct cell *c, int clear_sorts,
   /* ... and store. */
   // c->grav.ti_end_min = ti_gravity_end_min;
   // c->grav.ti_end_max = ti_gravity_end_max;
-  c->grav.ti_old_part = ti_current;
+  c->stars.ti_old_part = ti_current;
   c->stars.h_max = h_max;
 
   if (timer) TIMER_TOC(timer_dorecv_spart);