examples/transcoding: use av_malloc_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/ac3enc_float: use av_malloc_array()

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

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