[DEV] Fix aggro causing stunned creatures to face the wrong way.

NOTE:
  * 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.