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

LayoutTests/fast/canvas/canvas-large-pattern-expected.txt

Index: LayoutTests/fast/canvas/canvas-large-pattern-expected.txt
diff --git a/LayoutTests/fast/canvas/canvas-arc-connecting-line-expected.txt b/LayoutTests/fast/canvas/canvas-large-pattern-expected.txt
similarity index 56%
copy from LayoutTests/fast/canvas/canvas-arc-connecting-line-expected.txt
copy to LayoutTests/fast/canvas/canvas-large-pattern-expected.txt
index 9562be0d04265a63cccdedb8a59a60234c623de7..29ff3425b1b1770c24757007cbc643091622f788 100644
--- a/LayoutTests/fast/canvas/canvas-arc-connecting-line-expected.txt
+++ b/LayoutTests/fast/canvas/canvas-large-pattern-expected.txt
@@ -1,17 +1,20 @@
-Test that canvas arc()s are connected by a straight line. There should be two C-shapes with a line from the bottom of the left one to the top of the right one.
+Verifies createPattern using a source image that is a canvas 40k pixels wide.
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 PASS imgdata[0] is 0
-PASS imgdata[1] is 255
+FAIL imgdata[1] should be 255. Was 0.
 PASS imgdata[2] is 0
+FAIL imgdata[3] should be 255. Was 0.
 PASS imgdata[0] is 0
-PASS imgdata[1] is 255
+PASS imgdata[1] is 0
 PASS imgdata[2] is 0
+PASS imgdata[3] is 0
 PASS imgdata[0] is 0
 PASS imgdata[1] is 0
 PASS imgdata[2] is 0
+PASS imgdata[3] is 0
 PASS successfullyParsed is true
 
 TEST COMPLETE