Fix that last session could reset to default session

E.g. when opening and closing Qt Creator without opening a session.
The last session may only be updated to the "active" session, if it is
not the default-virgin session (which is actually a "no session" value).

Change-Id: If47da30d94c20df62d45960224e3137bcc6ba1b8
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/projectexplorer/projectexplorer.cpp

@@ -1556,6 +1556,7 @@ void ProjectExplorerPluginPrivate::savePersistentSettings()
     }
 
     QSettings *s = ICore::settings();
+    if (!SessionManager::isDefaultVirgin())
         s->setValue(QLatin1String("ProjectExplorer/StartupSession"), SessionManager::activeSession());
     s->remove(QLatin1String("ProjectExplorer/RecentProjects/Files"));