Propagate validation errors from inner readbuffer when deserializing picture. Also allow null paints.

BUG=skia:5812
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2379383002

Review-Url: https://codereview.chromium.org/2379383002
diff --git a/tests/SerializationTest.cpp b/tests/SerializationTest.cpp
index 24cab3f..9e40701 100644
--- a/tests/SerializationTest.cpp
+++ b/tests/SerializationTest.cpp
@@ -566,6 +566,7 @@
         // Deserialize picture
         SkValidatingReadBuffer reader(static_cast<void*>(data.get()), size);
         sk_sp<SkPicture> readPict(SkPicture::MakeFromBuffer(reader));
+        REPORTER_ASSERT(reporter, reader.isValid());
         REPORTER_ASSERT(reporter, readPict.get());
     }