Guard against nil player

https://gitlab.com/sztest/nodecore/-/issues/14
Unable to reproduce the crash, but the fix
seems obvious given the symptom.

mods/nc_player_pickup/dropall.lua

@@ -23,6 +23,7 @@ end
 
 local olddrop = minetest.item_drop
 function minetest.item_drop(item, player, ...)
+	if not player then return olddrop(item, player, ...) end
 	nodecore.player_discover(player, "item_drop")
 	local pctl = player:get_player_control()
 	if pctl.aux1 then