Avoid empty cells in the long-range gravity tasks.

src/engine.c

@@ -2321,6 +2321,10 @@ void engine_make_self_gravity_tasks(struct engine *e) {
   if (periodic)
     for (int i = 0; i < s->nr_cells; ++i) {
       const struct cell *c = &s->cells_top[i];
+      /* Skip empty cells */
+      if (c->gcount == 0) continue;
+
+      /* Did we correctly attach the FFT task ghosts? */
       if (c->nodeID == engine_rank &&
           (c->grav_ghost_in == NULL || c->grav_ghost_out == NULL))
         error("Invalid gravity_ghost for local cell");

src/runner_doiact_grav.h

@@ -1179,6 +1179,9 @@ void runner_do_grav_long_range(struct runner *r, struct cell *ci, int timer) {
     /* Avoid self contributions */
     if (ci == cj) continue;
 
+    /* Skip empty cells */
+    if (cj->gcount == 0) continue;
+
     /* Get the distance between the CoMs at the last rebuild*/
     double dx_r = CoM_rebuild_i[0] - multi_j->CoM_rebuild[0];
     double dy_r = CoM_rebuild_i[1] - multi_j->CoM_rebuild[1];