If a creature’s power is less than 0 when it’s doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. The same is true for its toughness. For example, if an effect has given Okaun -7/-0 so that its power and toughness are -4/3, doubling its power and toughness gives it -4/+3 and it’s -8/6 until end of turn.