QHeaderView: Fix crash in layout about to change

Before there was a risk looking up e.g index -1 if there
were no visible sections in layoutAboutToChange.

Change-Id: Ic911e4292e8e8c4892fef1c0f34cf7dccaad2bac
Task-number: QTBUG-65478
Reviewed-by: David Faure <david.faure@kdab.com>
(cherry picked from commit e8425f9e52c9df0ce0fbf122adff3ef6930f9961)

src/widgets/itemviews/qheaderview.cpp

@@ -2075,9 +2075,11 @@ void QHeaderViewPrivate::_q_layoutAboutToBeChanged()
     layoutChangePersistentSections.clear();
     layoutChangePersistentSections.reserve(std::min(10, sectionItems.count()));
     // after layoutChanged another section can be last stretched section
-    if (stretchLastSection) {
+    if (stretchLastSection && lastSectionLogicalIdx >= 0 && lastSectionLogicalIdx < sectionItems.count()) {
         const int visual = visualIndex(lastSectionLogicalIdx);
-        sectionItems[visual].size = lastSectionSize;
+        if (visual >= 0 && visual < sectionItems.size()) {
+            sectionItems[visual].size = lastSectionSize;
+        }
     }
     for (int i = 0; i < sectionItems.size(); ++i) {
         auto s = sectionItems.at(i);

tests/auto/widgets/itemviews/qheaderview/tst_qheaderview.cpp

@@ -2352,6 +2352,7 @@ void tst_QHeaderView::checkLayoutChangeEmptyModel()
     QtTestModel tm;
     tm.cols = 11;
     QTableView tv;
+    tv.verticalHeader()->setStretchLastSection(true);
     tv.setModel(&tm);
 
     const int section4Size = tv.horizontalHeader()->sectionSize(4) + 1;