Prevent punching with invalid userdata

Fixes for pandorabox-io/pandorabox.io#445

util.lua

@@ -325,6 +325,9 @@ local damage_def = {
 	damage_groups = {},
 }
 digtron.damage_creatures = function(player, source_pos, target_pos, amount, items_dropped)
+	if type(player) ~= 'userdata' then
+		return
+	end
 	local objects = minetest.env:get_objects_inside_radius(target_pos, 1.0)
 	if objects ~= nil then
 		damage_def.damage_groups.fleshy = amount
@@ -433,4 +436,4 @@ digtron.show_offset_markers = function(pos, offset, period)
 		local entity = safe_add_entity({x=buildpos.x, y=buildpos.y, z=z_pos + period}, "digtron:marker")
 		if entity ~= nil then entity:setyaw(1.5708) end
 	end
-end
+end