It avoids the overhead of the packet list; furthermore, using
ff_interleave_packet_per_dts() is wrong for the null muxer anyway,
because said muxer accepts packets without timestamps, which
ff_interleave_packet_per_dts() can't handle.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>