avcodec/mjpegdec: use av_mallocz_array()

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

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

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