buy: Fix bogus error when lot gets reused at the last prompt

buy: Fix bogus error when lot gets reused at the last prompt

When the lot being bid for goes away and gets reused while the player
is at the prompt for the destination sector, comm.com_amount gets
stale.  We use it before we detect the change and fail the command
This can lead to a misleading ""You don't have that much to spend!"
error.  Messed up when the code was fixed to deal with lot changes in
4.0.2.

Fix by checking for lot change earlier.

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