You can activate the scavenge ability, but on resolution it will not put any counters on the creature.
Rule 107.1b says
Most of the time, the Magic game uses only positive numbers and zero. You can’t choose a negative number, deal negative damage, gain negative life, and so on. However, it’s possible for a game value, such as a creature’s power, to be less than zero. If a calculation or comparison needs to use a negative value, it does so. If a calculation that would determine the result of an effect yields a negative number, zero is used instead, unless that effect doubles or sets to a specific value a player’s life total or the power and/or toughness of a creature or creature card.
In this case, the "calculation" is just plugging in the creature card's power, and the result is the number of +1/+1 counters to put on the target creature. If the power is negative, zero is used instead of the result, and no counters are added.