avfilter/af_silencedetect: use the name 's' for the pointer to the private context

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avfilter/vf_removelogo: use the name 's' for the pointer to the private context
ffmpeg: dont return reserved values

Found-by: Daemon404
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>