Do not recurse to lower cells within the sub-tasks if they don't contain any particles.

This should fix #274 (closed). @jwillis could you please check that your setup that crashes works with this version ?

It builds on your proposed fix but I moved the decision of aborting higher up in the recursion.

src/runner_doiact.h

@@ -1861,6 +1861,7 @@ void DOSUB_PAIR1(struct runner *r, struct cell *ci, struct cell *cj, int sid,
 
   /* Should we even bother? */
   if (!cell_is_active(ci, e) && !cell_is_active(cj, e)) return;
+  if (ci->count == 0 || cj->count == 0) return;
 
   /* Get the cell dimensions. */
   const float h = min(ci->width[0], min(ci->width[1], ci->width[2]));
@@ -2099,7 +2100,7 @@ void DOSUB_SELF1(struct runner *r, struct cell *ci, int gettimer) {
   TIMER_TIC;
 
   /* Should we even bother? */
-  if (!cell_is_active(ci, r->e)) return;
+  if (ci->count == 0 || !cell_is_active(ci, r->e)) return;
 
 #ifdef SWIFT_DEBUG_CHECKS
   cell_is_drifted(ci, r->e);
@@ -2153,6 +2154,7 @@ void DOSUB_PAIR2(struct runner *r, struct cell *ci, struct cell *cj, int sid,
 
   /* Should we even bother? */
   if (!cell_is_active(ci, e) && !cell_is_active(cj, e)) return;
+  if (ci->count == 0 || cj->count == 0) return;
 
   /* Get the cell dimensions. */
   const float h = min(ci->width[0], min(ci->width[1], ci->width[2]));
@@ -2391,7 +2393,7 @@ void DOSUB_SELF2(struct runner *r, struct cell *ci, int gettimer) {
   TIMER_TIC;
 
   /* Should we even bother? */
-  if (!cell_is_active(ci, r->e)) return;
+  if (ci->count == 0 || !cell_is_active(ci, r->e)) return;
 
   /* Recurse? */
   if (ci->split) {