dnxhdenc: fix mb_rc size

Fixes out of array access with RC_VARIANCE set to 0

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dnxhdenc: check that qmax is within the supported range

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_elbg: Fixed seed range of elbg filter

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>