decompress() trusted a caller-provided expected size (from the stored
block header, which has no checksum of its own) and allocated that many
bytes, up to 16 GB, before decoding. A corrupt size field could therefore
drive a huge allocation.
When the frame records its content size, the expected size must match
it. When it does not (streaming compressor output), decode incrementally
so the buffer grows with the real output and fail once it passes the
expected size.
