The only time we need to zero out scripts is for calcScriptHash which operates
on a deep copy anyway. This should make the tx passed to us unmodified now.
Use it to reduce code dpulication in the bip16 case.
In addition we export it so that that a user could run:
for !done && err == nil {
done, err = s.Step()
}
err = s.CheckErrorCondition()
manually instead of having to run Execute().
