Avoids implicit av_frame_ref() and therefore allocations
and error checks. It also avoids explicitly allocating
the AVFrames (done implicitly when getting the buffer).

It also fixes a data race: The AVFrame's sample_aspect_ratio
is currently updated after ff_thread_finish_setup()
and this write is unsynchronized with the read in av_frame_ref().
Removing the implicit av_frame_ref() fixed this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>