validate if fields sender is an actual player

fixes https://gitlab.com/4w/mtimer/-/issues/18

system/on_receive_fields.lua

@@ -7,6 +7,7 @@ local d = mtimer.dialog
 -- for this is very simple because most of the logic is handled in the
 -- timer functions and not in the formspec code.
 minetest.register_on_player_receive_fields(function(player, formname, fields)
+    if not player:is_player() then return end
     local meta = player:get_meta()
     local name = player:get_player_name()