avcodec/vc1dec: field pictures with direct mode MBs, followed by frame pictures are not supported

This case could occur when cuting and concatenating bitstreams

Fixes out of array read
Fixes: asan_heap-oob_1b33fdd_2849_cov_478905890_SA10143.vc1
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vc1dec.c

@@ -4342,6 +4342,10 @@ static void vc1_decode_b_mb_intfi(VC1Context *v)
             if (bmvtype == BMV_TYPE_DIRECT) {
                 dmv_x[0] = dmv_y[0] = pred_flag[0] = 0;
                 dmv_x[1] = dmv_y[1] = pred_flag[0] = 0;
+                if (!s->next_picture_ptr->field_picture) {
+                    av_log(s->avctx, AV_LOG_ERROR, "Mixed field/frame direct mode not supported\n");
+                    return;
+                }
             }
             vc1_pred_b_mv_intfi(v, 0, dmv_x, dmv_y, 1, pred_flag);
             vc1_b_mc(v, dmv_x, dmv_y, (bmvtype == BMV_TYPE_DIRECT), bmvtype);
@@ -6014,6 +6018,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
         goto err;
     }
 
+    v->s.current_picture_ptr->field_picture = v->field_mode;
     v->s.current_picture_ptr->f.interlaced_frame = (v->fcm != PROGRESSIVE);
     v->s.current_picture_ptr->f.top_field_first  = v->tff;