parse-util: in parse_permille() check negative earlier

parse-util: in parse_permille() check negative earlier

If 'v' is negative, it's wrong to add the decimal to it, as we'd
actually need to subtract it in this case. But given that we don't want
to allow negative vaues anyway, simply check earlier whether what we
have parsed so far was negative, and react to that before adding the
decimal to it.