Fix QDBusPlatformMenu::menuItemAt()

Other platform menu implementations check that the index is not out of
bounds, so make QDBusPlatformMenu::menuItemAt() do the same.

Change-Id: I1d2e42681f2dbfff8b80044141d620bddbc3b6aa
Task-number: QTBUG-51372
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>

src/platformsupport/dbusmenu/qdbusplatformmenu.cpp

@@ -262,7 +262,7 @@ void QDBusPlatformMenu::setContainingMenuItem(QDBusPlatformMenuItem *item)
 
 QPlatformMenuItem *QDBusPlatformMenu::menuItemAt(int position) const
 {
-    return m_items.at(position);
+    return m_items.value(position);
 }
 
 QPlatformMenuItem *QDBusPlatformMenu::menuItemForTag(quintptr tag) const