decompressor: add assertion about decompressed size
decompressor: add assertion about decompressed size

This is temporary until we support unknown output sizes.
indygreg/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,terrelln/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard,indygreg/python-zstandard