Prevent OOB reads for truncated H264 STAP-A packets.

BUG=webrtc:4771, webrtc:4834
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1238033003

Cr-Commit-Position: refs/heads/master@{#9650}

webrtc/modules/video_coding/main/source/session_info.cc

@@ -133,6 +133,8 @@ size_t VCMSessionInfo::InsertBuffer(uint8_t* frame_buffer,
 
   // We handle H.264 STAP-A packets in a special way as we need to remove the
   // two length bytes between each NAL unit, and potentially add start codes.
+  // TODO(pbos): Remove H264 parsing from this step and use a fragmentation
+  // header supplied by the H264 depacketizer.
   const size_t kH264NALHeaderLengthInBytes = 1;
   const size_t kLengthFieldLength = 2;
   if (packet.codecSpecificHeader.codec == kRtpVideoH264 &&