If the creature you control is an illegal target as Savage Smash tries to resolve, it won’t get +2/+2. If that creature is a legal target but the other creature isn’t, the creature you control still gets +2/+2.

You can’t cast Savage Smash 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 Smash resolves, neither creature will deal or be dealt damage.