WebRTC: h264 FU-A parse for both S and E flag

[chai51]

Describe the bug
In RTC3984, the FU-A format only explains the timing when the S flag and the E flag appear, so when there are both the S flag and the E flag in a FU-A packet, this is also allowed.

Version
Any version

To Reproduce
Steps to reproduce the behavior:

• Send a FU-A packet to SRS via WebRTC, with both the S and E flags set to 1.
• In the process of webrtc2rtmp: when SrsRtcFrameBuilder::packet_video_rtmp parses the RTP packet, the length of the NALU is not written; the NALUnitLength read by SrsFormat::do_avc_demux_ibmf_format is an invalid value.

Screenshots

[winlinvip]

Nice work! Would you please file an PullRequest to fix this issue?