buy: Don't continue when lot changes while asking for bid

When the lot being bid for changes while the player is at the "How
much" prompt, we report "Commodity #%d has changed!", and continue
with the changed lot.

If continuing is okay, we should keep quiet.  We did that until commit
40b11c098 "Fix buy not to wipe out concurrent updates", v4.3.27.  Okay
when only the lot's price changed.

However, the lot could have gone away, or even be reused for something
else.  Failing the command seems safest for the player, so do that.
It's how we use the check_FOO_ok() elsewhere.

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