Fix off-by-one error in DCHECK in ProducerFec.

webrtc/modules/rtp_rtcp/source/producer_fec_unittest.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 167 matching lines @@
                packet->header.header.timestamp,
                kRedPayloadType,
                packet->header.header.payloadType,
                red_packet.get(),
                true);  // Marker bit set.
   for (int i = 0; i < 10; ++i)
     EXPECT_EQ(i, red_packet->data()[kRtpHeaderSize + 1 + i]);
   delete packet;
 }
 
+TEST_F(ProducerFecTest, BuildRedPacketWithEmptyPayload) {
+  constexpr size_t kNumFrames = 1;
+  constexpr size_t kPayloadLength = 0;
+  constexpr size_t kRedForFecHeaderLength = 1;
+
+  generator_.NewFrame(kNumFrames);
+  std::unique_ptr<test::RawRtpPacket> packet(
+      generator_.NextPacket(0, kPayloadLength));
+  std::unique_ptr<RedPacket> red_packet =
+      ProducerFec::BuildRedPacket(packet->data, packet->length - kRtpHeaderSize,
+                                  kRtpHeaderSize, kRedPayloadType);
+  EXPECT_EQ(packet->length + kRedForFecHeaderLength, red_packet->length());
+  VerifyHeader(packet->header.header.sequenceNumber,
+               packet->header.header.timestamp, kRedPayloadType,
+               packet->header.header.payloadType, red_packet.get(),
+               true);  // Marker bit set.
+}
+
 }  // namespace webrtc