empires: Return immediately from reveal_next if we aren't in the REVEAL state

empires: Return immediately from reveal_next if we aren't in the REVEAL state

I think this fix came about after Kevin did some testing by mashing on
all of the host buttons as fast as possible.

It's definitely a correct fix as far as it's true we shouldn't be
revealing anything if we're not in the REVEAL state. But I think we're
due for a better audit of what's happening to various pieces of state
on state transitions, (such as, shouldn't the interval timer get
cleared before we _leave_ the REVEAL state).