I think sult is the West Norse form, Modern Swedish has svält. It's also possible that it's not a direct descendant of the original nominal form, but and independent creation from the verb, I guess. Thank you for your reply, anyway.
@Wakuran: Modern Swedish does have svält (Old Swedish svælter), which is indeed probably influenced by the verb form svälta (Old Swedish svælta); but older Swedish also has sylt (Old Swedish sulter, sylt and sylter) and Danish has sult. I have now discovered, however, that Swedish also has an obsolete svalt (Old Swedish svalter), which does seem to be < *swaltaz.