avfilter/vf_avgblur: Check plane instead of AVFrame

Fixes: CID1551694 Use after free (false positive based on assuming that out == in and one is freed and one used)

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit c296d4fdec)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_avgblur.c

@@ -287,7 +287,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
         const int width = s->planewidth[plane];
 
         if (!(s->planes & (1 << plane))) {
-            if (out != in)
+            if (out->data[plane] != in->data[plane])
                 av_image_copy_plane(out->data[plane], out->linesize[plane],
                                     in->data[plane], in->linesize[plane],
                                     width * ((s->depth + 7) / 8), height);