Assign correct script to EnemyBolt prefab

* was set to defunct Armer.cs
* now it's DestroyByContact.cs
Stop Enemy play EnemyBolt sound on own spawning
Check if GameController != null before ending game

gameController.GameOver() is called form Player's OnDestroy(), but
Player is also destroyed when user quits game without Player actually
dying. This means all objects are destroyed, possibly GameController
before Player.