xxan: fix invalid memory access in xan_decode_frame_type0()

The loop a few lines below the xan_unpack() call accesses up to
dec_size * 2 bytes into y_buffer, so dec_size must be limited to
buffer_size / 2.

CC:libav-stable@libav.org

libavcodec/xxan.c

@@ -308,7 +308,7 @@ static int xan_decode_frame_type0(AVCodecContext *avctx)
         int dec_size;
 
         bytestream2_seek(&s->gb, 8 + corr_off, SEEK_SET);
-        dec_size = xan_unpack(s, s->scratch_buffer, s->buffer_size);
+        dec_size = xan_unpack(s, s->scratch_buffer, s->buffer_size / 2);
         if (dec_size < 0)
             dec_size = 0;
         for (i = 0; i < dec_size; i++)