sweep/txgenerator: fix input witness ordering
sweep/txgenerator: fix input witness ordering

This commit fixes an issue that would arise if inputs without required
TxOuts would be swept together with inputs with required TxOuts. In this
case we would add the required ones first to the transaction, but did
not change the order we signed the inputs, resulting in signing the
wrong input index.

aakselrod/lnd,aakselrod/lnd