avcodec/smvjpegdec: check that frames_per_jpeg cleanly divides height

Fixes out of array access

If some valid files fail this check then please open a ticket and ping me and the smvjpegdec maintainer.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
