"Fix" another crash on valgrind bot

https://codereview.chromium.org/70463002/



git-svn-id: http://skia.googlecode.com/svn/trunk@12248 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/DeferredCanvasTest.cpp b/tests/DeferredCanvasTest.cpp
index a9679a3..bcee382 100644
--- a/tests/DeferredCanvasTest.cpp
+++ b/tests/DeferredCanvasTest.cpp
@@ -703,6 +703,10 @@
 #if SK_SUPPORT_GPU
     if (useGpu) {
         GrContext* context = factory->get(GrContextFactory::kNative_GLContextType);
+        if (NULL == context) {
+            return;
+        }
+
         surface = SkSurface::NewRenderTarget(context, imageSpec);
     } else {
         surface = SkSurface::NewRaster(imageSpec);