Only clear QLists when they are empty. Prevents crashes.

BUG: 182358

svn path=/trunk/KDE/kdebase/workspace/; revision=919007

effects/coverswitch.cpp

@@ -201,8 +201,10 @@ void CoverSwitchEffect::paintScreen( int mask, QRegion region, ScreenPaintData&
             rightIndex = 0;
 
         EffectWindow* frontWindow = tempList[ index ];
-        leftWindows.clear();
-        rightWindows.clear();
+        if( !leftWindows.isEmpty() )
+            leftWindows.clear();
+        if( !rightWindows.isEmpty() )
+            rightWindows.clear();
 
         bool evenWindows = ( tempList.count() % 2 == 0 ) ? true : false;
         int leftWindowCount = 0;