GMs: fail gracefully for null surface

A couple more instances where MakeRenderTarget returns nullptr
due to --preAbandonGpuContext.

BUG=skia5282
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1976163002

Review-Url: https://codereview.chromium.org/1976163002
diff --git a/gm/image.cpp b/gm/image.cpp
index a84e19b..b57ec3a 100644
--- a/gm/image.cpp
+++ b/gm/image.cpp
@@ -258,6 +258,7 @@
 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();
 }