Fixes#4374
It was possible for a broker to combine a sell and a buy offer from an account that already owns an NFT. Such brokering extracts money from the NFT owner and provides no benefit in return.
With this amendment, the code detects when a broker is returning an NFToken to its initial owner and prohibits the transaction. This forbids a broker from selling an NFToken to the account that already owns the token. This fixes a bug in the original implementation of XLS-20.
Thanks to @nixer89 for suggesting this fix.