fixed performance regression with ZSTD_decompress() on small files
fixed performance regression with ZSTD_decompress() on small files

memset() was a quick fix to initialization problems,
but initialize too much space (tables, buffers)
which show up in decompression speed of ZSTD_decompress()
since it needs to recreate DCtx at each invocation.

Fixed by only initialization relevant pointers and size fields.

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