vc1dec: check first field slices, fix out of array read.

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

libavcodec/vc1dec.c

@@ -5680,8 +5680,13 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
             s->start_mb_y = (i == 0) ? 0 : FFMAX(0, slices[i-1].mby_start % mb_height);
             if (!v->field_mode || v->second_field)
                 s->end_mb_y = (i == n_slices     ) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
-            else
+            else {
+                if (i >= n_slices) {
+                    av_log(v->s.avctx, AV_LOG_ERROR, "first field slice count too large\n");
+                    continue;
+                }
                 s->end_mb_y = (i <= n_slices1 + 1) ? mb_height : FFMIN(mb_height, slices[i].mby_start % mb_height);
+            }
             if (s->end_mb_y <= s->start_mb_y) {
                 av_log(v->s.avctx, AV_LOG_ERROR, "end mb y %d %d invalid\n", s->end_mb_y, s->start_mb_y);
                 continue;