avformat: add ff_alloc_extradata() helper

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avformat: use ff_alloc_extradata()

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avformat/westwood_vqa: s/unsigned char/uint8_t & s/unsigned int/uint32_t

Signed-off-by: Paul B Mahol <onemda@gmail.com>