avfilter/delogo: Check that logo area is inside the picture

We can only remove the logo if it is inside the picture. We need at
least one pixel around the logo area for interpolation.

Fixes ticket #5527 (Delogo crash with x=0 and/or y=0).

Signed-off-by: Jean Delvare <jdelvare@suse.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
