Freeciv

gtk-clients have

  if (*(dlg->source)) {
    *(dlg->source) = NULL;
  }

somewhere (depends on the client and branch) in their gui_stuff.c.
That condition makes no sense, as simply nullifying pointed variable would be only faster than first checking if it's already NULL and then nullifying.
I think the intention has been to check that dlg->source itself is not NULL, so we wouldn't try to nullify address 0.