Don't assume `hitter ~= nil` in on_punchplayer callback (#138)

* Don't assume `hitter ~= nil` in on_punchplayer callback

* Use early return

3d_armor/init.lua

@@ -394,6 +394,9 @@ end
 if armor.config.punch_damage == true then
 	minetest.register_on_punchplayer(function(player, hitter,
 			time_from_last_punch, tool_capabilities)
+		if hitter == nil then
+			return
+		end
 		local name = player:get_player_name()
 		local hit_ip = hitter:is_player()
 		if name and hit_ip and minetest.is_protected(player:get_pos(), "") then