QFormLayout: fix use-after-free in clearQLayoutItem()

Found by ASan when it should have been found by me in the initial
review...

The old code did, in this order:

1. delete item->layout() (which deletes item, as QLayoutItem::layout()
   is just a dynamic_cast implemented with virtual functions)

2. delete item->widget() (which is correct, but too late, as 'item'
   may have already been deleted; this is the first use-after-free
   bug)

3. delete item->spacerItem() (which is the second use-after-free).

Fix by deleting item->widget() (which may be a no-op), _then_ checking
for a QLayout, deleting all children (but not the layout), and finally
deleting item as a QLayoutItem.

Rename clearQLayoutItem() to clearAndDestroyQLayoutItem() to better
match what it actually does.

Change-Id: Id70a7a137dac5de466ef619f01bfd61dfc150418
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

0 files changed, 0 insertions, 0 deletions