Fix gcc conversion warning.

    size_t seed_start_position =

        negative_read2.find(read1 + si * seed_length, 0, seed_length);

    while (seed_start_position != std::string::npos &&

           read2_length - seed_start_position + seed_length * si >=

               (uint32_t)min_overlap_length &&

           seed_start_position >= si * seed_length) {

           (int)(read2_length - seed_start_position + seed_length * si) >=

               min_overlap_length &&

           seed_start_position >= (size_t)(si * seed_length)) {

      bool can_merge = true;

      int num_errors = 0;

      for (int i = 0; i < seed_length * si; ++i) {