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. For example, suppose you control Infesting Radroach, a 2/2 creature. If an effect has given Infesting Radroach -4/-0 so that it's a -2/2 creature, doubling its power gives it -2/-0, and it becomes a -4/2 creature.