xxan: check ybuf index before use.

Fixes out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/xxan.c

@@ -289,7 +289,8 @@ static int xan_decode_frame_type0(AVCodecContext *avctx)
             ybuf[j+1] = cur << 1;
             last = cur;
         }
-        ybuf[j] = last << 1;
+        if(j < avctx->width)
+            ybuf[j] = last << 1;
         prev_buf = ybuf;
         ybuf += avctx->width;
     }