The existing code would, incorrectly, allow negative amounts in offers
for non-fungible tokens. Such offers would be handled very differently
depending on the context: a direct offer would fail with an error code
indicating an internal processing error, whereas brokered offers would
improperly succeed.
This commit introduces the `fixNFTokenNegOffer` amendment that detects
such offers during creation and returns an appropriate error code.
The commit also extends the existing code to allow for buy offers that
contain a `Destination` field, so that a specific broker can be set in
the offer.
8266d9d598