Fixed crash in demo mode when demo mode is started while 'internet yes/no'

dialog is shown (or the track info dialog). Together with r11958 this fixed
#663.


git-svn-id: svn+ssh://svn.code.sf.net/p/supertuxkart/code/main/trunk@11960 178a84e3-b1eb-0310-8ba1-8eac791a3b58

src/modes/demo_world.cpp

@@ -18,6 +18,7 @@
 
 #include "modes/demo_world.hpp"
 
+#include "guiengine/modaldialog.hpp"
 #include "input/device_manager.hpp"
 #include "input/input_manager.hpp"
 #include "network/network_manager.hpp"
@@ -101,6 +102,10 @@ void DemoWorld::enterRaceOverState()
  */
 bool DemoWorld::updateIdleTimeAndStartDemo(float dt) 
 {
+    // We get crashes if stk is activated when a modal dialog is open
+    if(GUIEngine::ModalDialog::isADialogActive())
+        return false;
+
     m_current_idle_time += dt; 
     if(m_current_idle_time <= m_max_idle_time)
         return false;