avcodec/vc1dec: fix mby_start for interlaced content

Fixes first frame of Ticket2531
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -5858,7 +5858,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                                   buf_size3 << 3);
                     /* assuming that the field marker is at the exact middle,
                        hope it's correct */
-                    slices[n_slices].mby_start = s->mb_height >> 1;
+                    slices[n_slices].mby_start = s->mb_height + 1 >> 1;
                     n_slices1 = n_slices - 1; // index of the last slice of the first field
                     n_slices++;
                     break;
@@ -5909,7 +5909,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
                 buf_size3 = vc1_unescape_buffer(divider + 4, buf + buf_size - divider - 4, slices[n_slices].buf);
                 init_get_bits(&slices[n_slices].gb, slices[n_slices].buf,
                               buf_size3 << 3);
-                slices[n_slices].mby_start = s->mb_height >> 1;
+                slices[n_slices].mby_start = s->mb_height + 1 >> 1;
                 n_slices1 = n_slices - 1;
                 n_slices++;
             }