standalone: Reject bad input values in tx sanity.

[davecgh]

Transactions with input values that are negative or greater than the max supply ultimately will always eventually end up invalid by checks performed much later in the validation process. Moreover, the aforementioned conditions are entirely context free.

Given that, it is much more efficient and robust to simply reject any transactions that violate them as early as possible in the validation process.

The context-free transaction sanity checks are the ideal location since they are among the earliest validation checks that are performed.

However, unconfirmed transactions are allowed to leave the input value set to the special sentinel value of -1 (wire.NullValueIn) that signals the actual value will be filled in later. The sanity checks take place before that information is available to populate, so that case needs to be exempted and left for the later checks to reject as they already do now.

With that in mind, this modifies CheckTransactionSanity to reject transactions that violate those conditions accordingly.

It also adds ErrFraudAmountIn to uniquely identify when checks fail validation for that reason.

Finally, it modifies the rule error conversion in the internal blockchain code to recognize and convert the new error.

Additional tests are included in a second commit to test the new checks work as expected.