fix a crash (#190)

tube_registration.lua

@@ -109,7 +109,7 @@ local register_one_tube = function(name, tname, dropname, desc, plain, noctrs, e
 		after_place_node = pipeworks.after_place,
 		after_dig_node = pipeworks.after_dig,
 		on_blast = function(pos, intensity)
-			if intensity > 1 + 3^0.5 then
+			if not intensity or intensity > 1 + 3^0.5 then
 				minetest.remove_node(pos)
 				return {string.format("%s_%s", name, dropname)}
 			end