James Harvey points out that entering an invalid game ID can affect
the current midend state even if you don't subsequently enter a
valid one. Reorganise midend_game_id_int() so that (just like
midend_deserialise()) it does all its error checking before altering
anything in the midend's persistent data, so that it either succeeds
completely or fails before doing anything at all.

[originally from svn r6045]