Prevalidation without witness before witness required may stick.

[evoskuil]

Pre-validation without witness before witness required may cause the blockchain to perpetually reject a tx upon re-validation with witness forks. This because the witness data is additional missing required information which requires the node to re-obtain from a peer or the block under validation. In the latter case the block must have the witness, so there is no risk. Need to ensure that this scenario is covered and will be so with compact blocks as well.