Fix precision with non-ascii characters

[ShawnZhong]

Fix #3284

[vitaut]

Thanks for the PR but unfortunately it won't work with multibyte code points, e.g.

  fmt::print(">>{:.6}<<\n", "123ж56\xad");

It should print >>123ж56<< but would print >>123ж5<< with this change.

The correct fix is to handle invalid bytes as having width 1.