Qt: Fix Segfault when exiting on Windows 2000

Initialize two handles even if createTaskBarButtons() is not called.
Otherwise vlc.exe will segfault at exit
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/main_interface.cpp

@@ -169,6 +169,8 @@ MainInterface::MainInterface( intf_thread_t *_p_intf ) : QVLCMW( _p_intf )
     MainInputManager::getInstance( p_intf );
 
 #ifdef WIN32
+    himl = NULL;
+    p_taskbl = NULL;
     taskbar_wmsg = RegisterWindowMessage("TaskbarButtonCreated");
 #endif