During enemy invalidation reseting m_eEnemy to __NULL__ also set monster state to idle,
should be a task for NSMonster::SetEnemy() which is missed right now.
Add new NSMonster::IsValidEnemy(entity enemy) - returns TRUE if 'enemy' should be
considered a valid target for killing, visibility/reachability is not checked.
Add sanity checkes in NSMonster::WalkRoute() otherwise __NULL__ m_eEnemy could be used.
NSMonster_FindClosestPlayer() use IsValidEnemy() insead of health checking,
probably too strict and that changes its behaviour too much, probably.
ea462ea8ac