This is a fix/workaround for a special case that's caused by
https://github.com/btcsuite/btcd/blob/master/wire/msgtx.go#L426.
When a wire format transaction with no inputs is serialized, the wire
package assumes it's a non-witness transaction (as there is indeed no
witness data present).
But when de-serializing the same transaction, the line mentioned above
assumes that for the special case of a zero input length, the
transaction must be in the witness format, which causes the
de-serialization to fail.
The workaround in this commit fixes this special case by just trying
to deserialize the transaction in the non-witness format too.
