Remove path invalidation statement.

Caused problems on multiple detector rails in a row, because paths got cleared in-between a train step
Also optimize some code

advtrains/nodedb.lua

@@ -135,9 +135,6 @@ end
 function ndb.swap_node(pos, node, no_inval)
 	minetest.swap_node(pos, node)
 	ndb.update(pos, node)
-	if not no_inval then
-		advtrains.invalidate_all_paths(pos)
-	end
 end
 
 function ndb.update(pos, pnode)

advtrains/trainlogic.lua

@@ -433,7 +433,7 @@ function advtrains.train_step_b(id, train, dtime)
 
 	--hacky fix: if train_step_a returned in phase 2, end_index may not be set.
 	--just return
-	if not train.end_index then
+	if not train.index or not train.end_index then
 		return
 	end
 	

advtrains/wagons.lua

@@ -301,10 +301,10 @@ function wagon:on_step(dtime)
 		--check infotext
 		local outside=self:train().text_outside or ""
 		
-		local train = self:train()
+		local gp=self:train()
 		--show off-track information in outside text instead of notifying the whole server about this
-		local front_off_track=train.max_index_on_track and train.index and train.index>train.max_index_on_track
-		local back_off_track=train.min_index_on_track and train.end_index and train.end_index<train.min_index_on_track
+		local front_off_track=gp.max_index_on_track and gp.index and gp.index>gp.max_index_on_track
+		local back_off_track=gp.min_index_on_track and gp.end_index and gp.end_index<gp.min_index_on_track
 		if front_off_track or back_off_track then
 			outside = outside .."\n!!! Train off track !!!"
 		end
@@ -314,7 +314,6 @@ function wagon:on_step(dtime)
 			self.infotext_cache=outside
 		end
 		
-		local gp=self:train()
 		local fct=self.wagon_flipped and -1 or 1
 		--set line number
 		if self.name == "advtrains:subway_wagon" and gp.line and gp.line~=self.line_cache then