Fix compressed block size validation

[fallintoplace]

What

• Reject compressed blocks smaller than the 9-byte header.

Why

• The size comes from the wire.
• A value below 9 underflows compressed - HEADER_SIZE.
• This can make the payload read write past the temporary buffer.

Implementation

• Validate the lower bound before creating the temporary buffer.
• Add regression coverage for sizes 0, 1, and 8.