avfilter/vsrc_mandelbrot: use av_malloc_array()

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

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

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