xxan: more complete ybuf checks, fix out of array accesses.

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

libavcodec/xxan.c

@@ -276,7 +276,8 @@ 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;
 
@@ -350,7 +351,8 @@ static int xan_decode_frame_type1(AVCodecContext *avctx)
             ybuf[j+1] = cur;
             last = cur;
         }
-        ybuf[j] = last;
+        if(j < avctx->width)
+            ybuf[j] = last;
         ybuf += avctx->width;
     }