avcodec/xan: Improve overlapping check

Fixes: memcpy-param-overlap
Fixes: 3612/clusterfuzz-testcase-minimized-6393461273001984

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/xan.c

@@ -263,7 +263,7 @@ static inline void xan_wc3_copy_pixel_run(XanContext *s, AVFrame *frame,
     prevframe_index = (y + motion_y) * stride + x + motion_x;
     prevframe_x = x + motion_x;
 
-    if (prev_palette_plane == palette_plane && FFABS(curframe_index - prevframe_index) < pixel_count) {
+    if (prev_palette_plane == palette_plane && FFABS(motion_x + width*motion_y) < pixel_count) {
          avpriv_request_sample(s->avctx, "Overlapping copy");
          return ;
     }