GMs: fail gracefully for null surface

gm/image.cpp

Index: gm/image.cpp
diff --git a/gm/image.cpp b/gm/image.cpp
index a84e19b3f7758bc8c0017b183a37ca6b6b7b39aa..b57ec3a687e4b93c75ef3c27b2b97220945374e1 100644
--- a/gm/image.cpp
+++ b/gm/image.cpp
@@ -258,6 +258,7 @@ static sk_sp<SkImage> make_codec(const SkImageInfo& info, GrContext*, void (*dra
 static sk_sp<SkImage> make_gpu(const SkImageInfo& info, GrContext* ctx, void (*draw)(SkCanvas*)) {
     if (!ctx) { return nullptr; }
     auto surface(SkSurface::MakeRenderTarget(ctx, SkBudgeted::kNo, info));
+    if (!surface) { return nullptr; }
     draw(surface->getCanvas());
     return surface->makeImageSnapshot();
 }