"Fix" another crash in DeferredCanvasTest on valgrind bot

https://codereview.chromium.org/71463003/



git-svn-id: http://skia.googlecode.com/svn/trunk@12265 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/DeferredCanvasTest.cpp b/tests/DeferredCanvasTest.cpp
index bcee382..45d0292 100644
--- a/tests/DeferredCanvasTest.cpp
+++ b/tests/DeferredCanvasTest.cpp
@@ -775,6 +775,9 @@
 #if SK_SUPPORT_GPU
     if (useGpu) {
         GrContext* context = factory->get(GrContextFactory::kNative_GLContextType);
+        if (NULL == context) {
+            return;
+        }
         surface = SkSurface::NewRenderTarget(context, imageSpec);
         alternateSurface = SkSurface::NewRenderTarget(context, imageSpec);
     } else {