Escape the self-task recursion in the gravity sub-cell unskipping only if the...

Escape the self-task recursion in the gravity sub-cell unskipping only if the cell has *no* gparts not if it has *some* gparts.

src/cell.c

@@ -1870,7 +1870,7 @@ void cell_activate_subcell_grav_tasks(struct cell *ci, struct cell *cj,
   if (cj == NULL) {
 
     /* Do anything? */
-    if (ci->gcount || !cell_is_active_gravity(ci, e)) return;
+    if (ci->gcount == 0 || !cell_is_active_gravity(ci, e)) return;
 
     /* Recurse? */
     if (ci->split) {