Don't crash when calling popMenu() with empty stack.

Maybe quit button in main menu gets two click events while it's waiting for app destroy event.

src/guiengine/abstract_state_manager.cpp

@@ -209,6 +209,9 @@ void AbstractStateManager::popMenu()
 {
     assert(m_game_mode != GAME);
 
+    if (m_menu_stack.size() == 0)
+        return;
+
     // Send tear-down event to menu
     getCurrentScreen()->tearDown();
     m_menu_stack.pop_back();