avfilter/vf_spp: change temporary to unsigned

More consistent with uspp and allows for future 10bit support

Found-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avdevice/v4l2: use av_freep() to avoid leaving stale pointers in memory

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