x11font: fix handling of high-bit-set SBCS characters.

x11font: fix handling of high-bit-set SBCS characters.

I had mistakenly pulled a 'char' value out of a string and passed it
to x11_font_has_glyph and x11_char_struct, each of which takes its two
index bytes as int-typed parameters. But if chars are signed, that
turns high-bit-set characters into out-of-range array indices. Oops.

The range checks in x11_char_struct prevented that from causing any
problem worse than refusal to display any affected glyph. Even so,
that's not particularly helpful. Fixed by changing the index byte
parameters to unsigned char type.