fade: make draw_slice() chroma check against planes 1 and 2

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.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
