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.
Port fade filter from libavfilter soc repo, with minor fixes by Stefano. Signed-off-by: Michael Niedermayer <michaelni@gmx.at>