#501 Check credits after counter offer

[Anonymous]

If you attempt to trade when you do not have enough
credits for the initial offer, but do have enough if
the port agrees to the counter offer, you still get the
message that you do not have enough credits to trade.

Example:

Try to buy goods, you have 1799 credits. Port wants
1800 -- you never get to the counter offer screen --
you get the "error - you do not have enough credits"
message before you ever get to offer 1799