treat unknown node obstructing chest lid as solid

fixes #207

compat-chests.lua

@@ -44,6 +44,7 @@ local function chest_lid_obstructed(pos)
 	local above = { x = pos.x, y = pos.y + 1, z = pos.z }
 	local def = minetest.registered_nodes[minetest.get_node(above).name]
 	-- allow ladders, signs, wallmounted things and torches to not obstruct
+	if not def then return true end
 	if def.drawtype == "airlike" or
 			def.drawtype == "signlike" or
 			def.drawtype == "torchlike" or