avcodec/vc1_block: Simplify resetting coded_block

Everything that init_block_index() sets will be overwritten
a few lines below again, so don't call it and simply calculate
the only thing that is used (namely block_index[0]) manually.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/vc1_block.c

@@ -2680,8 +2680,7 @@ static int vc1_decode_i_blocks_adv(VC1Context *v)
     s->mb_x             = 0;
     s->mb_y             = s->start_mb_y;
     if (s->start_mb_y) {
-        init_block_index(v);
-        memset(&s->coded_block[s->block_index[0] - s->b8_stride], 0,
+        memset(&s->coded_block[(2 * s->mb_y - 1) * s->b8_stride - 2], 0,
                (1 + s->b8_stride) * sizeof(*s->coded_block));
     }
     for (; s->mb_y < s->end_mb_y; s->mb_y++) {