correctly compute the size of the maxexchange buffer. we put all fixes into one buffer.

  int onefix;

  maxexchange_fix = 0;

  for (int i = 0; i < nfix; i++) {

    onefix = fix[i]->maxexchange;

    maxexchange_fix = MAX(maxexchange_fix,onefix);

  }

  for (int i = 0; i < nfix; i++)

    maxexchange_fix += fix[i]->maxexchange;

  maxexchange = maxexchange_atom + maxexchange_fix;

  bufextra = maxexchange + BUFEXTRA;