fixed minor gcc warning
fixed minor gcc warning

gcc-8 on Linux doesn't like usage of strncat :
`warning: ‘strncat’ output truncated before terminating nul copying as many bytes from a string as its length`.

Not sure what was wrong, it might be a false positive,
but the logic is simple enough to replaced by a simple `memcpy()`,
thus avoiding the shenanigans of null-terminated strings.

Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd,Cyan4973/zstd