The layout for an invalid block is very likely to be null, it
shouldn't be accessed without checking the block's validity first.
We can make the check a bit more conservative and simply check that
the block isn't empty.
Change-Id: Ic1459a6168b1b8ce36e9c6d019dc28653676efbe
Task-number: QTBUG-43562
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>