Prevent graves from being destroyed by the same explosion that caused them

This means no more item/grave loss when a creeper kills a player.
The next explosion should be able to kill it, but currently is not likely because of high blast resistance.

Might also consider overloading dropBlockAsItemWithChance in BlockGrave and drop its contents manually. This way at least no items are destroyed when the grave is broken by an explosion.
Needs indentation.