h264parse: inserts extra NALU AUDs

In a pipeline demuxing an flv stream into h264 byte-stream, an additional NAL Access unit delimiter 0x0000000109f0 is inserted before the already existing AUD 0x000000010910 for every packet.

Tested with GStreamer 1.18.5 and main (1.19.2.1)

gst-launch-1.0 videotestsrc num-buffers=1 ! x264enc bitrate=1 ! flvmux ! flvdemux ! h264parse ! video/x-h264, stream-format=byte-stream ! fakesink dump=true

results in: 00000000 (0x7f2aa015c2d0): 00 00 00 01 09 f0 00 00 00 01 09 10 00 00 00 01 ................

gst-launch-1.0 videotestsrc num-buffers=1 ! x264enc bitrate=1 ! flvmux ! flvdemux ! h264parse ! video/x-h264 ! fakesink dump=true

results in: 00000000 (0x7f46ac0054e0): 00 00 00 02 09 10 00 00 00 1d 67 f4 00 0d 91 9b ..........g.....

the problem persists when working with an flv file that was generated by ffmpeg, so i suspect the culprit to be in the flv demuxer, not the muxer.