configure: (vp9) ensure bitstream compatibility

avoids picking up a library that could produce invalid bitstreams and
fixes build issues with the same.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg12dec: Don't create side data when current picture ptr is NULL.

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