decompressor: always call POOL_free()
decompressor: always call POOL_free()

Before, some code paths could lead to the pool leaking.
indygreg/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,terrelln/python-zstandard