Do not punch signs

For some reason, a sign text gets removed by a train when it drives by.
This should fix it.

advtrains/trainlogic.lua

@@ -598,7 +598,8 @@ function advtrains.train_step_b(id, train, dtime)
 			--- 8c damage other objects ---
 			local objs = minetest.get_objects_inside_radius(rcollpos, 2)
 			for _,obj in ipairs(objs) do
-				if not obj:is_player() and obj:get_armor_groups().fleshy and obj:get_armor_groups().fleshy > 0 then
+				if not obj:is_player() and obj:get_armor_groups().fleshy and obj:get_armor_groups().fleshy > 0 
+						and obj:get_luaentity() and obj:get_luaentity().name~="signs:text" then
 					obj:punch(obj, 1, { full_punch_interval = 1.0, damage_groups = {fleshy = 1000}, }, nil)
 				end
 			end