Handle the case where persistent indexes have gone invalid.

Don't add invalid ranges to the result. They will be removed
whenever d->ranges is processed for public consumption
anyway. For example, QItemSelectionModel::selection() calls
merge() with another selection. The merge() method removes
invalid ranges already. But the invalid ranges don't need
to be there in the first place, so this patch removes them.

A longer-term goal is to maintain d->ranges as an always-sorted list.

This method can be called with a vector containing invalid
QPersistentModelIndexes when those persistent indexes are made invalid
in between layoutAboutToBeChanged and layoutChanged. It's a normal
thing to happen and a case that should be handled deliberately.

Change-Id: I741ed9208d8a75644975c9e8d61f0d6d78e20576
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/itemmodels/qitemselectionmodel.cpp

@@ -853,12 +853,18 @@ static QItemSelection mergeIndexes(const QVector<QPersistentModelIndex> &indexes
     int i = 0;
     while (i < indexes.count()) {
         const QPersistentModelIndex &tl = indexes.at(i);
+        if (!tl.isValid()) {
+            ++i;
+            continue;
+        }
         QPersistentModelIndex br = tl;
         QModelIndex brParent = br.parent();
         int brRow = br.row();
         int brColumn = br.column();
         while (++i < indexes.count()) {
             const QPersistentModelIndex &next = indexes.at(i);
+            if (!next.isValid())
+                continue;
             const QModelIndex nextParent = next.parent();
             const int nextRow = next.row();
             const int nextColumn = next.column();