avcodec/dsddec: use av_malloc_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/cook: use av_malloc_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dvbsubdec: use av_mallocz_array()

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