Fix crash on exit when application does not use session management

This relates to Bug 1627

src/kernel/qapplication.cpp

@@ -1241,9 +1241,13 @@ QApplication::~QApplication()
 	qDebug( "Widgets left: %i    Max widgets: %i \n", QWidget::instanceCounter, QWidget::maxInstances );
     }
 #ifndef QT_NO_SESSIONMANAGER
-    delete session_manager;
+    if ( session_manager ) {
+	delete session_manager;
+    }
     session_manager = 0;
-    delete session_key;
+    if ( session_key ) {
+	delete session_key;
+    }
     session_key = 0;
 #endif //QT_NO_SESSIONMANAGER