oop: Fix nullptr dereference in a fuzz test case.

This patch does two things:
- Binds the test context to the thread, avoiding a dcheck
- Ensures to mark the reader as invalid if we couldn't read textblob
  data

R=ericrk@chromium.org

Bug: 794921
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I1cdf8a0c5571587b0d3a66f1a25d99ed5e3daed9
Reviewed-on: https://chromium-review.googlesource.com/827506Reviewed-by: Eric Karl <ericrk@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524152}

cc/paint/paint_op_buffer_fuzzer.cc

@@ -21,6 +21,7 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
       kRasterDimension, kRasterDimension, kOpaque_SkAlphaType);
   scoped_refptr<cc::TestContextProvider> context_provider =
       cc::TestContextProvider::Create();
+  context_provider->BindToCurrentThread();
   sk_sp<SkSurface> surface = SkSurface::MakeRenderTarget(
       context_provider->GrContext(), SkBudgeted::kYes, image_info);
   SkCanvas* canvas = surface->getCanvas();

cc/paint/paint_op_reader.cc

@@ -268,8 +268,10 @@ void PaintOpReader::Read(scoped_refptr<PaintTextBlob>* paint_blob,
                          TransferCacheDeserializeHelper* transfer_cache) {
   sk_sp<SkData> data;
   Read(&data);
-  if (!data || !valid_)
+  if (!data || !valid_) {
+    SetInvalid();
     return;
+  }
 
   // Skia expects the following to be true, make sure we don't pass it incorrect
   // data.