You can't cast Savage Stomp unless you choose both a creature you control and a creature you don't control as targets.

If either target is an illegal target as Savage Stomp resolves, neither creature will deal or be dealt damage.

If the creature you control is an illegal target as Savage Stomp tries to resolve, you won't put a +1/+1 counter on it. If that creature is a legal target but the other creature isn't, you'll still put the counter on the creature you control.