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.

unix/gtkfont.c

@@ -352,8 +352,8 @@ static int x11_font_width(XFontStruct *xfs, int sixteen_bit)
     }
 }
 
-static const XCharStruct *x11_char_struct(XFontStruct *xfs,
-                                          int byte1, int byte2)
+static const XCharStruct *x11_char_struct(
+    XFontStruct *xfs, unsigned char byte1, unsigned char byte2)
 {
     int index;
 
@@ -403,7 +403,8 @@ static const XCharStruct *x11_char_struct(XFontStruct *xfs,
     return &xfs->per_char[index];
 }
 
-static int x11_font_has_glyph(XFontStruct *xfs, int byte1, int byte2)
+static int x11_font_has_glyph(
+    XFontStruct *xfs, unsigned char byte1, unsigned char byte2)
 {
     /*
      * Not to be confused with x11font_has_glyph, which is a method of