Zero the new task pointers when rebuilding

src/space.c

@@ -398,6 +398,9 @@ void space_regrid(struct space *s, double cell_max, int verbose) {
       s->cells_top[k].extra_ghost = NULL;
       s->cells_top[k].ghost = NULL;
       s->cells_top[k].kick = NULL;
+      s->cells_top[k].grav_external = NULL;
+      s->cells_top[k].cooling = NULL;
+      s->cells_top[k].sourceterms = NULL;
       s->cells_top[k].super = &s->cells_top[k];
       s->cells_top[k].gsuper = &s->cells_top[k];
     }