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

Source/core/platform/graphics/Pattern.cpp

Index: Source/core/platform/graphics/Pattern.cpp
diff --git a/Source/core/platform/graphics/Pattern.cpp b/Source/core/platform/graphics/Pattern.cpp
index 61ae7ce3027ac6b9b68d418a047d36f3344d5fa5..3de80d91339eda5bb46258aa76715b525c9c4c2f 100644
--- a/Source/core/platform/graphics/Pattern.cpp
+++ b/Source/core/platform/graphics/Pattern.cpp
@@ -42,8 +42,9 @@ Pattern::Pattern(PassRefPtr<Image> image, bool repeatX, bool repeatY)
     , m_repeatY(repeatY)
     , m_externalMemoryAllocated(0)
 {
-    ASSERT(image);
-    m_tileImage = image->nativeImageForCurrentFrame();
+    if (image) {
+        m_tileImage = image->nativeImageForCurrentFrame();
+    }
 }
 
 Pattern::~Pattern()