Clean up abandoned spaceships

When a player leaves, his spaceship is left behind, lurking
around the data and taking space in the block. Clean up this
debris by checking whether the spaceship has its driver
still present and removing it if not.

mods/saturn/player.lua

@@ -617,6 +617,9 @@ local spaceship = {
 
 function spaceship:on_step(dtime)
 	self.age = self.age + dtime
+	if not self.driver then
+		self.object:remove()
+	end
 	if self.driver and self.driver:get_look_dir() then
 		local player = self.driver
 		local name = player:get_player_name()