Add missing type check to InvRef:set_lists()

[Zughy]

• Goal of the PR
  Avoiding a segmentation fault crash (AKA the whole client)
• How does the PR work?
  I've added a table check
• Does it resolve any reported issue?
  One of the problems in Many Lua C functions are missing type checks #1794. I'd like to fix them all but I thought about starting with one

To do

This PR is Ready for Review.

How to test

minetest.register_on_joinplayer(function(player, last_login)
  minetest.after(1, function()
    player:get_inventory():set_lists()
  end)
end)

the client won't crash anymore (but the game will, of course)