Fix crash when a player happens to join the server quicker than the creative...

Fix crash when a player happens to join the server quicker than the creative inventory filler is called

mods/creative/init.lua

 -- minetest/creative/init.lua
 
 local creative_inventory = {}
+creative_inventory.creative_inventory_size = 0
 
 -- Create detached creative inventory after loading all mods
 minetest.after(0, function()