Fix for destroying a tilemarker.

If you try to destroy activated tilemarker (with red laser), minecraft crash with the next stacktrace:
```
java.lang.NullPointerException
	at buildcraft.builders.TileMarker.destroy(TileMarker.java:369)
	at buildcraft.builders.TileMarker.func_70313_j(TileMarker.java:328)
	at net.minecraft.world.chunk.Chunk.func_76627_f(Chunk.java:1058)
	at net.minecraft.world.World.func_72932_q(World.java:2925)
	at net.minecraft.world.chunk.Chunk.func_76592_a(Chunk.java:722)
	at net.minecraft.world.World.func_72832_d(World.java:554)
	at net.minecraft.world.World.func_94571_i(World.java:683)
	at net.minecraft.block.Block.removeBlockByPlayer(Block.java:1629)
	at net.minecraft.client.multiplayer.PlayerControllerMP.func_78751_a(PlayerControllerMP.java:154)
	at net.minecraft.client.multiplayer.PlayerControllerMP.func_78744_a(PlayerControllerMP.java:81)
	at net.minecraft.client.multiplayer.PlayerControllerMP.func_78743_b(PlayerControllerMP.java:193)
	at net.minecraft.client.Minecraft.func_71402_c(Minecraft.java:1305)
	at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1791)
	at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:831)
	at net.minecraft.client.Minecraft.run(Minecraft.java:756)
	at java.lang.Thread.run(Thread.java:722)
```