GeezerVison

Fixes #265.

The --print-bytes branch of format_verbose_difference already pads with saturating_sub; the --verbose branch a few lines down uses a plain -. Since offset_width comes from the smaller file's metadata length, a file that reports length 0 but still yields bytes collapses it to 2, and the subtraction underflows as soon as the running offset reaches three digits.

$ printf '\377%.0s' {1..150} > nz
$ cmp --verbose /dev/zero nz

Before, that stops after byte 99 (abort under overflow-checks, a huge padding loop otherwise). After, all 150 lines print and it exits 1.

Made the two branches match rather than adding a separate guard.

Worth flagging since the issue shows GNU's output: the column is still narrower than GNU's here, because offset_width is derived from a metadata length that is 0 for this kind of file. That is the same root input, but it is a formatting difference rather than a crash, so I have left it out of this change.

Tests: an integration test covering the case, gated to unix for /dev/zero.