avcodec/roqvideoenc: use av_malloc(z)_array()

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

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

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