message: fuzz: make H2 codec fuzz test more robust to unexpected new streams.
  Previously, mutations that generated new streams that we weren't expecting could cause the fuzzer
  itself to dereference a null pointers in ON_CALL(newStream).
  Fixes oss-fuzz issue
  Risk level: Low
  Testing: Corpus entry added.
diff: @@ -273,9 +273,14 @@ DEFINE_PROTO_FUZZER(const test::common::http::http2::CodecImplFuzzTestCase& inpu
  // the response encoder and can complete Stream initialization.
  std::list<StreamPtr> pending_streams;
  std::list<StreamPtr> streams;
  + // For new streams when we aren't expecting one (e.g. as a result of a mutation).
  + NiceMock<MockStreamDecoder> orphan_request_decoder;
  ON_CALL(server_callbacks, newStream(_))
  .WillByDefault(Invoke([&](StreamEncoder& encoder) -> StreamDecoder& {
  + if (pending_streams.empty()) {
  + return orphan_request_decoder;
  + }
  auto stream_ptr = pending_streams.front()->removeFromList(pending_streams);
  Stream* const stream = stream_ptr.get();
  stream_ptr->moveIntoListBack(std::move(stream_ptr), streams);
  