Verifier for FlexBuffers (#6977)

* Verifier for FlexBuffers

* Verifier improvements & fuzzer

tests/fuzzer/flexbuffers_verifier_fuzzer.cc

@@ -0,0 +1,14 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+#include <stddef.h>
+#include <stdint.h>
+#include <string>
+
+#include "flatbuffers/flexbuffers.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  std::vector<bool> reuse_tracker;
+  flexbuffers::VerifyBuffer(data, size, &reuse_tracker);
+  return 0;
+}