Fix assertion failure if you Undo right at startup.

Fix assertion failure if you Undo right at startup.

The initial call to midend_new_game() was creating a partial
serialisation containing no game states at all, which meant that if
your first UI action was an undo operation, the game would try to
deserialise that and complain that it was incomplete. Now we detect
that in advance and don't create a useless serialisation in the first
place.