Enhance game initialization so that creepers never spawn on the house or Ferris

[darioalessandro]

Check this method:

escape-the-creepers-with-rust/src/model/mod.rs

Lines 124 to 172 in 6a7d38c

	GameEvents::InitGameWithCreepers(creepers, rows, columns) => {
	// spawn creepers
	let mut randy = thread_rng();
	let creepers = (0..creepers)
	.into_iter()
	.map(|_i| {
	let row = randy.gen_range(0..rows);
	let column = randy.gen_range(0..columns);
	Creeper {
	location: Location { x: row, y: column },
	}
	})
	.collect();
	// TODO: validate that Ferris does not spawn next or on top of a creeper.
	let row = randy.gen_range(0..rows);
	let column = randy.gen_range(0..columns);
	let ferris = Ferris {
	location: Location { x: row, y: column },
	path: vec![],
	};
	let row = randy.gen_range(0..rows);
	let column = randy.gen_range(0..columns);
	let target = Location { x: row, y: column };
	let moves = vec![GameState { creepers, ferris }];
	let mut game = Game {
	rows: rows,
	columns: columns,
	moves,
	target,
	status: Status::Idle,
	};
	let origin = &game.moves.last().unwrap().ferris.location;
	let target = &game.target;
	let result = Dijkstra::run(&game, origin, &target, &Mode::Ferris);
	match result {
	Ok(result) => {
	// This unwrap will work because we just added a move.
	let mut ferris = &mut game.moves.last_mut().unwrap().ferris;
	ferris.path = result;
	game.validate_status();
	game.into()
	}
	Err(err) => {
	game.status = Status::Error(format!("{}", err));
	game.into()
	}
	}
	}

Ensure that creepers never spawn on the house game.target or ferris game.moves.last().ferris.location