QTableWidget: avoid complete redrawing when an item is removed

When an item gets removed from QTableWidget, the dataChanged() signal
is sent out with an invalid index. This triggers a complete repaint
which is not needed since only one cell is changed.
Fix it by retrieving the model index *before* the internal structure is
cleaned for the given item since otherwise index() will no longer find
the item and return an invalid index.

Change-Id: Ia0d82edb6b44118e8a1546904250ca29d9c45140
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/widgets/itemviews/qtablewidget.cpp

@@ -247,8 +247,8 @@ void QTableModel::removeItem(QTableWidgetItem *item)
 {
     int i = tableItems.indexOf(item);
     if (i != -1) {
-        tableItems[i] = 0;
         QModelIndex idx = index(item);
+        tableItems[i] = nullptr;
         emit dataChanged(idx, idx);
         return;
     }