QTreeWidget: Fix UB (member call)

Before actually deleting QTreeWidgetItems from QTree{Model,Widget{,Item}} dtors,
their 'view' members need to be set to nullptr, lest they attempt to delist
themselves from the list of top-level items.

For the QTreeModel::headerItem, this was forgottten.

Found by UBSan:

  qtreewidget.cpp:1488:70: runtime error: member call on address 0x7ffd843dd470 which does not point to an object of type 'QAbstractItemView'
  0x7ffd843dd470: note: object is of type 'QWidget'
    #0 0x2b83d5b48323 in QTreeWidgetItem::~QTreeWidgetItem() src/widgets/itemviews/qtreewidget.cpp:1488
    #1 0x2b83d5b48860 in QTreeWidgetItem::~QTreeWidgetItem() src/widgets/itemviews/qtreewidget.cpp:1535
    #2 0x2b83d5b41659 in QTreeModel::~QTreeModel() src/widgets/itemviews/qtreewidget.cpp:143
    #3 0x2b83d5b41bc0 in QTreeModel::~QTreeModel() src/widgets/itemviews/qtreewidget.cpp:146
    #4 0x2b83df220747 in QObjectPrivate::deleteChildren() src/corelib/kernel/qobject.cpp:2010
    #5 0x2b83d4603dd0 in QWidget::~QWidget() src/widgets/kernel/qwidget.cpp:1675
    qt#6 0x2b83d4d76066 in QFrame::~QFrame() src/widgets/widgets/qframe.cpp:256
    qt#7 0x2b83d5270442 in QAbstractScrollArea::~QAbstractScrollArea() src/widgets/widgets/qabstractscrollarea.cpp:575
    qt#8 0x2b83d5733eb9 in QAbstractItemView::~QAbstractItemView() src/widgets/itemviews/qabstractitemview.cpp:617
    qt#9 0x2b83d598b216 in QTreeView::~QTreeView() src/widgets/itemviews/qtreeview.cpp:206
    qt#10 0x2b83d5b218b6 in QTreeWidget::~QTreeWidget() src/widgets/itemviews/qtreewidget.cpp:2549
    qt#11 0x4eef42 in tst_QTreeWidgetItemIterator::updateIfModifiedFromWidget() tests/auto/widgets/itemviews/qtreewidgetitemiterator/tst_qtreewidgetitemiterator.cpp:1089

Change-Id: I57c277adee8c99eb07b274d6d8ea1f6fbf3575be
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/itemviews/qtreewidget.cpp

@@ -140,6 +140,7 @@ QTreeModel::QTreeModel(QTreeModelPrivate &dd, QTreeWidget *parent)
 QTreeModel::~QTreeModel()
 {
  clear();
+ headerItem->view = Q_NULLPTR;
  delete headerItem;
  rootItem->view = 0;
  delete rootItem;