lib/fold: Catch error case from mbrtowc()

The assert() statement in fold_text() only evaluates in a debug build.
If mbrtowc() encounters an error return the portion of the string that
has been parsed and stop.

This avoids an issue with glibc 2.22 where previous calls to
setlocale() failed and set an unsuitable locale. Since the error was not
caught this resulted in an infinite loop when trying to access the
Language screen.

Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>