This is required for those containers which mux the
AVCDecoderConfigurationRecord as private/extra data
so that the AVC packetizer works correctly.

This should close #2850
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 80308100)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>