Bug 630465. Don't pretend everything is ok when allocating image surfaces failes. r=vlad, a=joe

content/canvas/src/nsCanvasRenderingContext2D.cpp

@@ -1164,7 +1164,7 @@ nsCanvasRenderingContext2D::InitializeWithSurface(nsIDocShell *docShell, gfxASur
     // then the surface became cleared, and we need to redraw everything.
     Redraw();
 
-    return NS_OK;
+    return mValid ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
 }
 
 NS_IMETHODIMP