Function message_at 

fn message_at(
    path: &Path,
    buffer: &Buffer,
    offset: usize,
    body_len: usize,
) -> Result<Framed>

Frame the message at offset, bounds-checking every number it declares against the region the CRC covers.

The checks are not belt-and-braces. Buffer::slice_with_length panics on a range it does not like, panic = "abort" makes that a process death, and the numbers being checked were read off a disk — see corrupt.