Add a safety check to desktop grid for the case that you click on an area where there is no desktop.

That case needs some more improvement as it zooms to the black area and suddenly replaces the screen with the actual windows.
BUG: 219721

svn path=/trunk/KDE/kdebase/workspace/; revision=1065215

effects/desktopgrid/desktopgrid.cpp

@@ -809,6 +809,8 @@ EffectWindow* DesktopGridEffect::windowAt( QPoint pos ) const
 
     int desktop;
     pos = unscalePos( pos, &desktop );
+    if( desktop >= effects->numberOfDesktops() )
+        return NULL;
     if( isUsingPresentWindows() )
         {
         const int screen = effects->screenNumber( pos );