Previously, the "allow EndOfStream" part of this logic was too permissive. If there are a few dangling bytes at the end of the stream, that should be treated as a bad magic number. The only case where EndOfStream is allowed is when the stream is truly at the end, with exactly zero bytes available.
This eliminates a footgun and special case handling with fixed buffers,
as well as allowing decompression streams to keep a window in the output
buffer.
This passes tests but it doesn't provide as big a window size as is
required to decompress larger streams.
The next commit in this branch will work towards that, without
introducing an additional buffer.
