matroskadec: fix out of bounds write

Signed-off-by: Janne Grunau <janne-libav@jannau.net>
(cherry picked from commit 723229c11f)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit d51c7b4cbe)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -1641,7 +1641,7 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
                         lace_size[n] = lace_size[n - 1] + snum;
                         total += lace_size[n];
                     }
-                    lace_size[n] = size - total;
+                    lace_size[laces - 1] = size - total;
                     break;
                 }
             }