Fix glitch in tube autorouter

(brain-o in how I was handling unknown/bad nodes being passed to it.  this
works better and lets the code do its job properly).

autoplace_tubes.lua

@@ -17,11 +17,7 @@ end
 
 --a function for determining which side of the node we are on
 local function nodeside(node, tubedir)
-	if not tubedir or
-	   not node or
-	   not (type(node.param2) == "number" and node.param2 > 0 and node.param2 < 23) then
-		return "back"
-	end
+	if node and (node.param2 < 0 or node.param2 > 23) then node.param2 = 0 end
 
 	--get a vector pointing back
 	local backdir = minetest.facedir_to_dir(node.param2)