avcodec/h264_slice: Check picture structure before setting the related fields

This might fix a hypothetical race condition
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_slice.c

@@ -1285,7 +1285,7 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
     int needs_reinit = 0;
     int field_pic_flag, bottom_field_flag;
     int first_slice = h == h0 && !h0->current_slice;
-    int frame_num;
+    int frame_num, picture_structure, droppable;
     PPS *pps;
 
     h->qpel_put = h->h264qpel.put_h264_qpel_pixels_tab;
@@ -1503,39 +1503,35 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
             return AVERROR_INVALIDDATA;
         }
     }
-    h->frame_num = frame_num;
 
     h->mb_mbaff        = 0;
     h->mb_aff_frame    = 0;
     last_pic_structure = h0->picture_structure;
     last_pic_droppable = h0->droppable;
-    h->droppable       = h->nal_ref_idc == 0;
+    droppable          = h->nal_ref_idc == 0;
     if (h->sps.frame_mbs_only_flag) {
-        h->picture_structure = PICT_FRAME;
+        picture_structure = PICT_FRAME;
     } else {
         if (!h->sps.direct_8x8_inference_flag && slice_type == AV_PICTURE_TYPE_B) {
             av_log(h->avctx, AV_LOG_ERROR, "This stream was generated by a broken encoder, invalid 8x8 inference\n");
             return -1;
         }
         field_pic_flag = get_bits1(&h->gb);
+
         if (field_pic_flag) {
             bottom_field_flag = get_bits1(&h->gb);
-            h->picture_structure = PICT_TOP_FIELD + bottom_field_flag;
+            picture_structure = PICT_TOP_FIELD + bottom_field_flag;
         } else {
-            h->picture_structure = PICT_FRAME;
+            picture_structure = PICT_FRAME;
             h->mb_aff_frame      = h->sps.mb_aff;
         }
     }
-    h->mb_field_decoding_flag = h->picture_structure != PICT_FRAME;
-
-    if (h0->current_slice != 0) {
-        if (last_pic_structure != h->picture_structure ||
-            last_pic_droppable != h->droppable) {
+    if (h0->current_slice) {
+        if (last_pic_structure != picture_structure ||
+            last_pic_droppable != droppable) {
             av_log(h->avctx, AV_LOG_ERROR,
                    "Changing field mode (%d -> %d) between slices is not allowed\n",
                    last_pic_structure, h->picture_structure);
-            h->picture_structure = last_pic_structure;
-            h->droppable         = last_pic_droppable;
             return AVERROR_INVALIDDATA;
         } else if (!h0->cur_pic_ptr) {
             av_log(h->avctx, AV_LOG_ERROR,
@@ -1543,7 +1539,14 @@ int ff_h264_decode_slice_header(H264Context *h, H264Context *h0)
                    h0->current_slice + 1);
             return AVERROR_INVALIDDATA;
         }
-    } else {
+    }
+
+    h->picture_structure = picture_structure;
+    h->droppable         = droppable;
+    h->frame_num         = frame_num;
+    h->mb_field_decoding_flag = picture_structure != PICT_FRAME;
+
+    if (h0->current_slice == 0) {
         /* Shorten frame num gaps so we don't have to allocate reference
          * frames just to throw them away */
         if (h->frame_num != h->prev_frame_num) {