QAbstractItemView: do not request illegal model indexes

If a model is empty, there is no index (0, 0); requesting it
is undefined behavior.

Rather than protecting the calls with checks on rowCount/columnCount,
remove the Q_ASSERTs altogether: they're trying to do some
basic sanity checks on the model, something that doesn't belong
to a view (but, say, to QAbstractItemModelTester).

Change-Id: I0ea25604fdcf524a10f5922a03a4d0700447f6a7
Reviewed-by: Andre Somers <andre.somers@kdab.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/itemviews/qabstractitemview.cpp

@@ -705,15 +705,6 @@ void QAbstractItemView::setModel(QAbstractItemModel *model)
     }
     d->model = (model ? model : QAbstractItemModelPrivate::staticEmptyModel());
 
-    // These asserts do basic sanity checking of the model
-    Q_ASSERT_X(d->model->index(0,0) == d->model->index(0,0),
-               "QAbstractItemView::setModel",
-               "A model should return the exact same index "
-               "(including its internal id/pointer) when asked for it twice in a row.");
-    Q_ASSERT_X(!d->model->index(0,0).parent().isValid(),
-               "QAbstractItemView::setModel",
-               "The parent of a top level index should be invalid");
-
     if (d->model != QAbstractItemModelPrivate::staticEmptyModel()) {
         connect(d->model, SIGNAL(destroyed()),
                 this, SLOT(_q_modelDestroyed()));