secure m_highlightedWindows access when triggering

explicit repaint in highlight effect

forward port of 891a0a2a3272186c2c7a93f7a332e04809e3d4d4

effects/highlightwindow/highlightwindow.cpp

@@ -240,7 +240,8 @@ void HighlightWindowEffect::prepareHighlighting()
     foreach (EffectWindow * w, effects->stackingOrder())
     if (!m_windowOpacity.contains(w))    // Just in case we are still finishing from last time
         m_windowOpacity[w] = isInitiallyHidden(w) ? 0.0 : 1.0;
-    m_highlightedWindows.at(0)->addRepaintFull();
+    if (!m_highlightedWindows.isEmpty())
+        m_highlightedWindows.at(0)->addRepaintFull();
 }
 
 void HighlightWindowEffect::finishHighlighting()