vc1dec: Do not allow field_mode to change after the first header

This fixes out of array accesses.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/vc1.c

@@ -825,6 +825,7 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
     int status;
     int mbmodetab, imvtab, icbptab, twomvbptab, fourmvbptab; /* useful only for debugging */
     int scale, shift, i; /* for initializing LUT for intensity compensation */
+    int field_mode, fcm;
 
     v->p_frame_skipped = 0;
     if (v->second_field) {
@@ -836,19 +837,23 @@ int ff_vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
             goto parse_common_info;
     }
 
-    v->field_mode = 0;
+    field_mode = 0;
     if (v->interlace) {
-        v->fcm = decode012(gb);
-        if (v->fcm) {
-            if (v->fcm == ILACE_FIELD)
-                v->field_mode = 1;
+        fcm = decode012(gb);
+        if (fcm) {
+            if (fcm == ILACE_FIELD)
+                field_mode = 1;
             if (!v->warn_interlaced++)
                 av_log(v->s.avctx, AV_LOG_ERROR,
                        "Interlaced frames/fields support is incomplete\n");
         }
     } else {
-        v->fcm = PROGRESSIVE;
+        fcm = PROGRESSIVE;
     }
+    if (!v->first_pic_header_flag && v->field_mode != field_mode)
+        return AVERROR_INVALIDDATA;
+    v->field_mode = field_mode;
+    v->fcm = fcm;
 
     if (v->field_mode) {
         v->fptype = get_bits(gb, 3);

libavcodec/vc1.h

@@ -372,6 +372,7 @@ typedef struct VC1Context{
     int qs_last;            ///< if qpel has been used in the previous (tr.) picture
     int bmvtype;
     int frfd, brfd;         ///< reference frame distance (forward or backward)
+    int first_pic_header_flag;
     int pic_header_flag;
 
     /** Frame decoding info for sprite modes */

libavcodec/vc1dec.c

@@ -5463,6 +5463,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
 
     // do parse frame header
     v->pic_header_flag = 0;
+    v->first_pic_header_flag = 1;
     if (v->profile < PROFILE_ADVANCED) {
         if (ff_vc1_parse_frame_header(v, &s->gb) < 0) {
             goto err;
@@ -5472,6 +5473,7 @@ static int vc1_decode_frame(AVCodecContext *avctx, void *data,
             goto err;
         }
     }
+    v->first_pic_header_flag = 0;
 
     if ((avctx->codec_id == AV_CODEC_ID_WMV3IMAGE || avctx->codec_id == AV_CODEC_ID_VC1IMAGE)
         && s->pict_type != AV_PICTURE_TYPE_I) {