MFC: xkb: Escape non-printable characters correctly

XkbStringText escapes non-printable characters using octal numbers.
Such escape sequence would be at most 5 characters long ("\0123"), so
it reserves 5 bytes in the buffer. Due to char->unsigned int
conversion, it would print much longer string for negative numbers.

xserver/xkb/xkbtext.c

@@ -603,7 +603,7 @@ XkbStringText(char *str, unsigned format)
             }
             else {
                 *out++ = '0';
-                sprintf(out, "%o", *in);
+                sprintf(out, "%o", (unsigned char) *in);
                 while (*out != '\0')
                     out++;
             }