avcodec/vp56: use av_reallocp_array() and check if allocation failed

Signed-off-by: Paul B Mahol <onemda@gmail.com>
doc: Rename avtools-common-opts to fftools-common opts

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>