Unbreak libxo's handling of characters not representable in current locale

[kan]

Summary

The xo_format_string_direct function loops forever never advancing the processed string pointer
when it encounters a character that makes mbrtowc fail. Make it emit '?' character instead, as
it seems this is what the code intent was, sans bugs.

Test Plan

Create file in UTF-8 locate, try to ls directory with that file under, say. KOI8-R locale. Observe ls getting unhappy without the patch. Observe it printing '?' with the patch.

[eadler]

this seams reasonable to me

[marcel]

LGTM. I'll forward to Phil.