budget: Fix level prod. forecast when required level is too low

budget: Fix level prod. forecast when required level is too low

When the required level is too low for production, produce() returns
early.  Except when simulating.  Messed up when Empire 3 made the
update code work for budget.

This can make budget show level production even when it's not actually
possible.  In the stock game, this can happen for tech and research,
which require education > 5.0.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>