zstd_opt.h: small improvement in compression ratio
zstd_opt.h: small improvement in compression ratio

ds77/zstd,ds77/zstd,Majlen/zstd,Cyan4973/zstd,ds77/zstd,ds77/zstd,inikep/zstd,Cyan4973/zstd,Majlen/zstd,Majlen/zstd,Cyan4973/zstd,Majlen/zstd,Cyan4973/zstd,inikep/zstd,inikep/zstd,Cyan4973/zstd,inikep/zstd