detect when something tries to delete a callback that was never added

{

  if (id == NULL) return;

  int ifix;

  for (ifix = 0; ifix < modify->nfix; ifix++)

    if (strcmp(id,modify->fix[ifix]->id) == 0) break;

  int ifix = modify->find_fix(id);

  // compact the list of callbacks

    int match;

    for (match = 0; match < nextra_grow; match++)

      if (extra_grow[match] == ifix) break;

    if ((nextra_grow == 0) || (match == nextra_grow))

      error->all(FLERR,"Trying to delete non-existent Atom::grow() callback");

    for (int i = match; i < nextra_grow-1; i++)

      extra_grow[i] = extra_grow[i+1];

    nextra_grow--;

    int match;

    for (match = 0; match < nextra_restart; match++)

      if (extra_restart[match] == ifix) break;

    if ((nextra_restart == 0) || (match == nextra_restart))

      error->all(FLERR,"Trying to delete non-existent Atom::restart() callback");

    for (int i = match; i < nextra_restart-1; i++)

      extra_restart[i] = extra_restart[i+1];

    nextra_restart--;

    int match;

    for (match = 0; match < nextra_border; match++)

      if (extra_border[match] == ifix) break;

    if ((nextra_border == 0) || (match == nextra_border))

      error->all(FLERR,"Trying to delete non-existent Atom::border() callback");

    for (int i = match; i < nextra_border-1; i++)

      extra_border[i] = extra_border[i+1];

    nextra_border--;