Simulators don't simulate in invalid chunks (valgrind error)

git-svn-id: http://mc-server.googlecode.com/svn/trunk@1676 0a769ca7-a7f5-676a-18bf-c427514a06d6

source/Simulator/DelayedFluidSimulator.cpp

@@ -86,7 +86,7 @@ void cDelayedFluidSimulator::AddBlock(int a_BlockX, int a_BlockY, int a_BlockZ,
 		return;
 	}
 	
-	if (a_Chunk == NULL)
+	if ((a_Chunk == NULL) || !a_Chunk->IsValid())
 	{
 		return;
 	}