Fix two nul deref if digging unknown nodes. (#5398)

Introduced by d785456 (#5162)

builtin/game/item.lua

@@ -496,15 +496,15 @@ function core.node_dig(pos, node, digger)
	core.handle_node_drops(pos, drops, digger)

	local oldmetadata = nil
	if def.after_dig_node then
	if def and def.after_dig_node then
		oldmetadata = core.get_meta(pos):to_table()
	end

	-- Remove node and update
	core.remove_node(pos)

	-- Run callback
	if def.after_dig_node then
	if def and def.after_dig_node then
		-- Copy pos and node because callback can modify them
		local pos_copy = {x=pos.x, y=pos.y, z=pos.z}
		local node_copy = {name=node.name, param1=node.param1, param2=node.param2}