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

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

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/jpeg2000dwt: use av_malloc_array()

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