replace hard crash with disabling atom sorting, when no suitable binsize is found

  // user setting if explicitly set

  // default = 1/2 of neighbor cutoff

  // check if neighbor cutoff = 0.0

  // and in that case, disable sorting

  double binsize;

  if (userbinsize > 0.0) binsize = userbinsize;

  else binsize = 0.5 * neighbor->cutneighmax;

  if (binsize == 0.0) error->all(FLERR,"Atom sorting has bin size = 0.0");

  else if (neighbor->cutneighmax > 0.0) binsize = 0.5 * neighbor->cutneighmax;

  if ((binsize == 0.0) && (sortfreq > 0)) {

    sortfreq = 0;

    if (comm->me == 0)

          error->warning(FLERR,"No pairwise cutoff or binsize set. "

                         "Atom sorting therefore disabled.");

    return;

  }

  double bininv = 1.0/binsize;