Resolve an issue in updating candidate cache when the position is negative.

      int size = cache[hidx].positive_candidates.size();

      int shift = (int)minimizers[0].second >> 1;

      for (i = 0; i < size; ++i)

        cache[hidx].positive_candidates[i].position += shift;

      {

        int rid = (int)(cache[hidx].positive_candidates[i].position >> 32);

        int rpos = (int)cache[hidx].positive_candidates[i].position;

        cache[hidx].positive_candidates[i].position = ((uint64_t)rid << 32) + rpos + shift;

      }

      size = cache[hidx].negative_candidates.size();

      for (i = 0; i < size; ++i)

        cache[hidx].negative_candidates[i].position -= shift;