hash_128_above can only cover half string for hash compute

[wymhuster]

static LJ_NOINLINE uint32_t hash_128_above(uint64_t seed, const char* str,
					   uint32_t len)
{
  uint32_t chunk_num, chunk_sz, chunk_sz_log2, i, pos1, pos2;
  uint64_t h1, h2, v;
  const char* chunk_ptr;

  chunk_num = 16;
  chunk_sz = len / chunk_num;
  chunk_sz_log2 = log2_floor(chunk_sz);

  pos1 = get_random_pos_unsafe(chunk_sz_log2, 0);
  pos2 = get_random_pos_unsafe(chunk_sz_log2, 1);

  h1 = lj_crc32_u32(0, len ^ seed);
  h2 = 0;

  /* loop over 14 chunks, 2 chunks at a time */
  for (i = 0, chunk_ptr = str; i < (chunk_num / 2 - 1);
       chunk_ptr += chunk_sz, i++) {

    v = *cast_uint64p(chunk_ptr + pos1);
    h1 = lj_crc32_u64(h1, v);

    v = *cast_uint64p(chunk_ptr + chunk_sz + pos2);
    h2 = lj_crc32_u64(h2, v);
  }

every loop just move one chunk，it's a bug here? the loop only cover half string
should we set chunk_ptr += 2*chunk_sz
#60

[wymhuster]

@zhuizhuhaomeng @agentzh