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

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

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

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