fix no-gui crash if no firmware was installed

It's a bit mysterious, but the confirmation box doesn't like being called in this mode.
So let's just return false.

rpcs3/rpcs3qt/gui_application.cpp

@@ -358,6 +358,7 @@ void gui_application::InitializeCallbacks()
 
 	callbacks.on_missing_fw = [this]()
 	{
+		if (!m_main_window) return false;
 		return m_gui_settings->GetBootConfirmation(m_main_window, gui::ib_confirm_fw);
 	};