Similar to the previous commit which applied to QCborStreamReader, don't
allocate too much data before checking that the stream actually has that
much.
Pick-to: 5.15 6.0
Fixes: QTBUG-88256
Change-Id: I7b9b97ae9b32412abdc6fffd16454b7568a063ba
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>