When linking the hierarchical gravity tasks, do not link to the parents if we...

When linking the hierarchical gravity tasks, do not link to the parents if we are at the super-level.

src/engine_maketasks.c

@@ -1115,9 +1115,19 @@ void engine_link_gravity_tasks(struct engine *e) {
     const enum task_types t_type = t->type;
     const enum task_subtypes t_subtype = t->subtype;
 
-    struct cell *ci_parent = (ci->parent != NULL) ? ci->parent : ci;
-    struct cell *cj_parent =
-        (cj != NULL && cj->parent != NULL) ? cj->parent : cj;
+    /* Pointers to the parent cells for tasks going up and down the tree
+     * In the case where we are at the super-level we don't
+     * want the parent as no tasks are defined above that level. */
+    struct cell *ci_parent, *cj_parent;
+    if (ci->parent != NULL && ci->grav.super != ci)
+      ci_parent = ci->parent;
+    else
+      ci_parent = ci;
+
+    if (cj != NULL && cj->parent != NULL && cj->grav.super != cj)
+      cj_parent = cj->parent;
+    else
+      cj_parent = cj;
 
 /* Node ID (if running with MPI) */
 #ifdef WITH_MPI