Use ZSTD_DCtx in simple decompress
Use ZSTD_DCtx in simple decompress

The function always creates a ZSTD_DCtx but previously only used
it for dictionary decompression. Internally, ZSTD_decompress()
allocated another ZSTD_DCtx. So skip ZSTD_decompress() and use
ZSTD_decompressDCtx() directly with the explicitly allocated
ZSTD_DCtx.
terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard