message: Fix RtpFrameReferenceFinderFuzzer to not generate invalid input
  Make sure that the packets in the packet buffer belonging to the
  first and last sequence numbers are marked as first and last,
  respectively.
  Cr-Commit-Position: refs/heads/master@{#28769}
diff: @@ -123,9 +123,16 @@ void FuzzOneInput(const uint8_t* data, size_t size) {
  video_coding::RtpFrameReferenceFinder reference_finder(&cb);
  while (reader.MoreToRead()) {
  + // Make sure that these packets fulfill the contract of RtpFrameObject.
  + uint16_t first_seq_num = reader.GetNum<uint16_t>();
  + uint16_t last_seq_num = reader.GetNum<uint16_t>();
  + VCMPacket* first_packet = pb->GetPacket(first_seq_num);
  + VCMPacket* last_packet = pb->GetPacket(last_seq_num);
  + first_packet->video_header.is_first_packet_in_frame = true;
  + last_packet->video_header.is_last_packet_in_frame = true;
  +
  auto frame = absl::make_unique<video_coding::RtpFrameObject>(
  - pb, reader.GetNum<uint16_t>(), reader.GetNum<uint16_t>(), 0, 0, 0, 0,
  - RtpPacketInfos());
  + pb, first_seq_num, last_seq_num, 0, 0, 0, 0, RtpPacketInfos());
  reference_finder.ManageFrame(std::move(frame));
  }
  }
  