mxfdec: check edit_rate also for physical_track

Previously only the edit_rate of material_track was checked.
If it's negative, it causes assertion failures in av_rescale_rnd.

Reviewed-by: Tim Nicholson <nichot20-at-yahoo.com@ffmpeg.org>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
