avcodec/a64multienc: use av_malloc(z)_array()

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

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

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