Fix crash due to missing null check in squashing code

Source/core/rendering/RenderLayerCompositor.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 1117 matching lines @@
                 layer->setGroupedMapping(0);
                 layer->setShouldInvalidateNextBacking(true);
             }
         }
     }
 
     // At this point, the layer's compositingState() is correct and indicates where this layer would paint into. Issue
     // an invalidation if we needed to.
     if (layer->shouldInvalidateNextBacking()) {
         // FIXME: these should invalidate only the rect that needs to be invalidated, i.e. the layer's rect in the squashingLayer's space.
-        if (layer->compositingState() == PaintsIntoGroupedBacking && squashingState.mostRecentMapping->squashingLayer())
+        if (layer->compositingState() == PaintsIntoGroupedBacking && squashingState.mostRecentMapping->squashingLayer()) {
             squashingState.mostRecentMapping->squashingLayer()->setNeedsDisplay();
-        else
-            layer->enclosingCompositingLayerForRepaint(ExcludeSelf)->compositedLayerMapping()->setContentsNeedDisplay();
+        } else {
+            RenderLayer* enclosingLayer = layer->enclosingCompositingLayerForRepaint(ExcludeSelf);
+            if (enclosingLayer->hasCompositedLayerMapping())
+                enclosingLayer->compositedLayerMapping()->setContentsNeedDisplay();
+        }
 
         layer->setShouldInvalidateNextBacking(false);
     }
 
     if (layer->stackingNode()->isStackingContainer()) {
         RenderLayerStackingNodeIterator iterator(*layer->stackingNode(), NegativeZOrderChildren);
         while (RenderLayerStackingNode* curNode = iterator.next())
             assignLayersToBackingsInternal(curNode->layer(), squashingState, layersChanged);
     }
 
@@ skipping 1501 matching lines @@
     } else if (graphicsLayer == m_scrollLayer.get()) {
         name = "Frame Scrolling Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace WebCore