fixed a bug in saving index

  std::cerr << "Sorted minimizers.\n";

  uint32_t num_minimizers = tmp_table.size();

  assert(num_minimizers != 0 && num_minimizers <= INT_MAX); // Here I make sure the # minimizers is less than the limit of signed int32, so that I can use int to store position later.

  kh_resize(k64, lookup_table_, num_minimizers);

  occurrence_table_.reserve(num_minimizers);

  uint64_t previous_key = tmp_table[0].first;

  uint32_t num_previous_minimizer_occurrences = 0;