add 2 value sanity checks which should not be needed but are for any reason

game crashed because of these values being nil, I don't know how these values could even become nil, but whatever.
Interestingly, I never found the wagon entity that was causing the crashs

wagons.lua

@@ -174,6 +174,12 @@ end
 function wagon:on_step(dtime)
 	local t=os.clock()
 	local pos = self.object:getpos()
+	
+	if not pos then
+		print("["..self.unique_id.."][fatal] missing position (object:getpos() returned nil)")
+		return
+	end
+	
 	if not self.initialized_pre then 
 		print("[advtrains] wagon stepping while not yet initialized_pre, returning")
 		self.object:setvelocity({x=0,y=0,z=0})
@@ -269,6 +275,10 @@ function wagon:on_step(dtime)
 		self.object:setvelocity({x=0, y=0, z=0})
 		return
 	end
+	if not self.pos_in_train then
+		print("["..self.unique_id.."][fatal] no pos_in_train set.")
+		return
+	end
 	
 	local index=advtrains.get_real_path_index(self:train(), self.pos_in_train)
 	--print("trainindex "..gp.index.." wagonindex "..index)