Fix aggro causing stunned creatures to face the wrong way.

If a creature is aggroed with a stun, the creature will begin chasing the
player and get stunned in the same update, in that order. So when the stun
causes the creature to stop moving, it will have it's orientation turned
toward the player.

To fix it, creatures shouldn't get a new orientation in Unit::StopMoving
when they are currently stunned.