[XeTeX] handling malformed UTF-8 input

[Akira Kakuto]

> For those who are happy rebuilding xetex from source, I'd appreciate
> knowing of any problems with your (real) UTF-8 files after this patch
> is applied. As far as I know, valid files should be processed unchanged.

The new one fails to create ConTeXt format:
It stops when it is reading 'lang-cz.pat' with an
error message '!Nonletter'. Probably 'lang-cz.pat'
is not a utf-8 file.

Best regards,
Akira