[libzstd] Set CLEVEL_CUSTOM correctly
[libzstd] Set CLEVEL_CUSTOM correctly

In `ZSTD_compressBegin_advanced()`, `ZSTD_parameters` are used to set the
compression parameters, but the level didn't get set to `CLEVEL_CUSTOM`, so
`ZSTD_compressBlock()` used the wrong parameters when checking the source
size.

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