fix btowc corner case

btowc is required to interpret its argument by conversion to unsigned
char, unless the argument is equal to EOF. since the conversion to
produces a non-character value anyway, we can just unconditionally
convert, for now.

src/multibyte/btowc.c

@@ -3,5 +3,6 @@
 
 wint_t btowc(int c)
 {
+	c = (unsigned char)c;
 	return c<128U ? c : EOF;
 }