avfilter/vf_libopencv: use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vsrc_life: Use av_freep() avoid leaving stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mpegenc: replace always enabled asserts by av_assert0()

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