When the display matrix is not the identity one, but the rotation angle
is zero, there is no need to update the sample aspect ratio.

Otherwise, it is possible to obtain negative values which interferes
with transcoding in later stages. This kind of behaviour is reproducible
on mov files with "major_brand: MSNV".

CC: libav-stable@libav.org
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>