Fix crash in 2D canvas drawSystemFocusRing with invalid argument.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012, 2013 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 2220 matching lines @@
     // override this for accessibility, but currently Blink doesn't take advantage of this.
     return element->focused();
 }
 
 bool CanvasRenderingContext2D::focusRingCallIsValid(const Path& path, Element* element)
 {
     if (!state().m_invertibleCTM)
         return false;
     if (path.isEmpty())
         return false;
-    if (!element->isDescendantOf(canvas()))
+    if (!element || !element->isDescendantOf(canvas()))
         return false;
 
     return true;
 }
 
 void CanvasRenderingContext2D::updateFocusRingAccessibility(const Path& path, Element* element)
 {
     if (!canvas()->renderer())
         return;
 
@@ skipping 45 matching lines @@
     const int focusRingWidth = 5;
     const int focusRingOutline = 0;
     c->drawFocusRing(path, focusRingWidth, focusRingOutline, focusRingColor);
 
     c->restore();
 
     didDraw(dirtyRect);
 }
 
 } // namespace WebCore