Segfault on calculating GISAS with external data

In !1578 (merged) in JobItem::cropRealData() after replacing

std::unique_ptr<Datafield> origData(intensityItem->c_field()->clone());

with

const Datafield* origData = intensityItem->c_field();

we got segfault on trying to access already deleted data: (*origData)[it.detectorIndex()];