tuklib_mbstr_width: Use stricter return value checking

This should make no difference in practice (at least if mbrtowc()
isn't broken).

src/common/tuklib_mbstr_width.c

@@ -41,7 +41,7 @@ tuklib_mbstr_width(const char *str, size_t *bytes)
 	while (i < len) {
 		wchar_t wc;
 		const size_t ret = mbrtowc(&wc, str + i, len - i, &state);
-		if (ret < 1 || ret > len)
+		if (ret < 1 || ret > len - i)
 			return (size_t)-1;
 
 		i += ret;