Harden record parser helpers against offset+length overflow

[Yeounui]

Hello. Thank you for this project.

The record parsing helpers do the bounds check as 'size >= offset + length'.
If an offset or length (both size_t) is very large, the sum can overflow and wrap around, so the check passes and the parser reads out of bounds.

Suggested fix: 'offset <= size && length <= size - offset'.
Not reachable from the packet path (record length is a checked uint16_t); this just guards the helpers against a caller passing a bad offset or length, e.g. from unchecked size_t arithmetic.

I also added a small CTest regression for a near-SIZE_MAX offset.