xxan: don't read before start of buffer in av_memcpy_backptr().

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/xxan.c

@@ -129,7 +129,8 @@ static int xan_unpack(uint8_t *dest, const int dest_len,
                 if (size + size2 > dest_end - dest)
                     break;
             }
-            if (src + size > src_end || dest + size + size2 > dest_end)
+            if (src + size > src_end || dest + size + size2 > dest_end ||
+                dest - orig_dest + size < back)
                 return -1;
             bytestream_get_buffer(&src, dest, size);
             dest += size;