draw_slice() checks that the fade factor is < 65536 and only
calculates the fade if so. But the fade factor is clipped in
end_frame() by av_clip_uint16() to 65535, so the fade is calculated
for every frame. This patch alters the check so that it compares with
< 65535 (UINT16_MAX).
draw_slice() checks that planes 0 and 1 of AVFilterBufferRef's data
are not NULL before manipulating planes 1 and 2. This patch makes the
check against planes 1 and 2. More senseful and possibly more robust.