avcodec/vc1dec: use av_realloc_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/tta: use av_malloc(z)_array()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/snow: use av_malloc(z)_array()

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