Zeroing hash only for xxhash

For xxhash one use only first 64 bit from 256 bit hash field
Zeroing empty 192 bits with offset

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Delete useless memset before XXH64_reset

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>