qt-quick: initialize prev and next when adding chart items

The code assumed that when adding chart items to lists, prev
and next are initialized to null. Make this more robust.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

qt-quick/chartview.cpp

@@ -235,11 +235,13 @@ void ChartView::ChartItemList::remove(ChartItem &item)
 void ChartView::ChartItemList::append(ChartItem &item)
 {
 	if (!first) {
+		item.prev = nullptr;
 		first = &item;
 	} else {
 		item.prev = last;
 		last->next = &item;
 	}
+	item.next = nullptr;
 	last = &item;
 }