avcodec/cngdec: use av_mallocz_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/nut: add ff_nut_audio_extra_tags to demuxer too

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpegvideo_enc: change some asserts to av_asserts

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