Check RTP sequence number order when assembling H264 frame in PacketBuffer

PacketBuffer::buffer_ slots can be re-used by packets with with
different sequence numbers when their difference is multiple of
buffer_.size()

Non-H264 code path validates RTP sequence numbers continuity in
`PotentialNewFrame` helper, H264 code path missed such check as the new
test demonstrate.

Bug: chromium:537233963
Change-Id: I6af0f52bfcb3a4437450dffcece85bee7ce172cd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/490840
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48222}

modules/video_coding/packet_buffer.cc

diff --git a/modules/video_coding/packet_buffer.cc b/modules/video_coding/packet_buffer.cc
index 961b41e..0efaa87 100644
--- a/modules/video_coding/packet_buffer.cc
+++ b/modules/video_coding/packet_buffer.cc
@@ -339,6 +339,8 @@
         // See: https://bugs.chromium.org/p/webrtc/issues/detail?id=7106
         if (is_h264_descriptor &&
             (buffer_[start_index] == nullptr ||
+             buffer_[start_index]->seq_num() !=
+                 static_cast<uint16_t>(start_seq_num - 1) ||
              buffer_[start_index]->timestamp != frame_timestamp)) {
           break;
         }

modules/video_coding/packet_buffer_unittest.cc

diff --git a/modules/video_coding/packet_buffer_unittest.cc b/modules/video_coding/packet_buffer_unittest.cc
index 29f13e4..45b09e6 100644
--- a/modules/video_coding/packet_buffer_unittest.cc
+++ b/modules/video_coding/packet_buffer_unittest.cc
@@ -709,6 +709,14 @@
   InsertH264(4662, kKeyFrame, kFirst, kLast, 1000);
 }
 
+TEST_P(PacketBufferH264ParameterizedTest, InsertMisOrderedPackets) {
+  InsertH264(4660, kKeyFrame, kFirst, kNotLast, 1000);
+  // packet (4661 + kStartSize) can use the same buffer slot as packet 4661,
+  // PacketBuffer need to be careful to detect they are not the same packet.
+  InsertH264(4661 + kStartSize, kDeltaFrame, kFirst, kNotLast, 1000);
+  InsertH264(4662, kKeyFrame, kFirst, kLast, 1000);
+}
+
 TEST_P(PacketBufferH264ParameterizedTest, ClearMissingPacketsOnKeyframe) {
   InsertH264(0, kKeyFrame, kFirst, kLast, 1000);
   InsertH264(2, kKeyFrame, kFirst, kLast, 3000);