Don't assert for NULL instance in killInstance

Singleton pattern is about not carring about this.
Fixing failing assert when quitting VLC after the view menu has been
clicked
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/dialogs/extensions.hpp

@@ -75,7 +75,6 @@ public:
     }
     static void killInstance()
     {
-        assert( instance != NULL );
         delete instance;
         instance = NULL;
     }