Fix potential integer overflow and unbounded allocation in ReadBlock

[Noamismach]

Description

While reviewing the SSTable parsing logic, I noticed that the varint-decoded handle.size() in ReadBlock is cast to size_t and added to kBlockTrailerSize without prior bounds checking or overflow validation.

In scenarios where a corrupted or malformed .ldb file is read, an excessively large block size can trigger an integer overflow (n + kBlockTrailerSize < n), or force the new char[] allocator to request an enormous amount of heap memory, leading to an immediate std::bad_alloc abort (process crash).

Fix

This patch hardens the parser by:

1. Adding an integer overflow guard before the trailer addition.
2. Introducing a safe maximum block size cap (256MB) for both compressed and decompressed block payloads.
3. Returning Status::Corruption instead of allowing the application to crash, ensuring higher robustness.

All existing tests pass successfully.

[Noamismach]

Hi team,
Just a gentle ping on this PR. Let me know if there's anything else you need from my side to approve the workflows or if any adjustments are required for the patch.

Thanks!