roqvideodec: check dimensions validity

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 3ae610451170cd5a28b33950006ff0bd23036845)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit fee26d352a52eb9f7fcd8d9167fb4a5ba015b612)

CC: libav-stable@libav.org

Signed-off-by: Reinhard Tartler <siretart@tauware.de>
