[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.

lib/compress/zstd_compress.c

@@ -240,6 +240,7 @@ static ZSTD_CCtx_params ZSTD_assignParamsToCCtxParams(
     ZSTD_CCtx_params ret = cctxParams;
     ret.cParams = params.cParams;
     ret.fParams = params.fParams;
+    ret.compressionLevel = ZSTD_CLEVEL_CUSTOM;
     return ret;
 }