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>
