configure: fix libavfilter pkgconfig description

Signed-off-by: Paul B Mahol <onemda@gmail.com>
vc1dec: dont decode slices when the slice header failed to decode.

Fixes assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>