[2D Canvas] Fix crash in createPattern method when using invalid canvas as source.

Source/core/platform/graphics/Pattern.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2013 Google, 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.
@@ skipping 24 matching lines @@
 #include "core/platform/graphics/Image.h"
 #include "core/platform/graphics/skia/SkiaUtils.h"
 
 namespace WebCore {
 
 Pattern::Pattern(PassRefPtr<Image> image, bool repeatX, bool repeatY)
     : m_repeatX(repeatX)
     , m_repeatY(repeatY)
     , m_externalMemoryAllocated(0)
 {
-    ASSERT(image);
-    m_tileImage = image->nativeImageForCurrentFrame();
+    if (image) {
+        m_tileImage = image->nativeImageForCurrentFrame();
+    }
 }
 
 Pattern::~Pattern()
 {
     if (m_externalMemoryAllocated)
         v8::V8::AdjustAmountOfExternalAllocatedMemory(-m_externalMemoryAllocated);
 }
 
 SkShader* Pattern::shader()
 {
@@ skipping 37 matching lines @@
 }
 
 void Pattern::setPatternSpaceTransform(const AffineTransform& patternSpaceTransformation)
 {
     m_patternSpaceTransformation = patternSpaceTransformation;
     if (m_pattern)
         m_pattern->setLocalMatrix(affineTransformToSkMatrix(m_patternSpaceTransformation));
 }
 
 }