avcodec/h264_slice: Dont reset mb_aff_frame per slice

Fixes null pointer dereference
Fixes Ticket4440
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_slice.c

@@ -1198,6 +1198,7 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
     int field_pic_flag, bottom_field_flag;
     int first_slice = sl == h->slice_ctx && !h->current_slice;
     int frame_num, picture_structure, droppable;
+    int mb_aff_frame, last_mb_aff_frame;
     PPS *pps;
 
     h->qpel_put = h->h264qpel.put_h264_qpel_pixels_tab;
@@ -1433,12 +1434,13 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
     }
 
     sl->mb_mbaff       = 0;
-    h->mb_aff_frame    = 0;
+    last_mb_aff_frame  = h->mb_aff_frame;
     last_pic_structure = h->picture_structure;
     last_pic_droppable = h->droppable;
     droppable          = h->nal_ref_idc == 0;
     if (h->sps.frame_mbs_only_flag) {
         picture_structure = PICT_FRAME;
+        mb_aff_frame      = 0;
     } 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");
@@ -1451,12 +1453,13 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
             picture_structure = PICT_TOP_FIELD + bottom_field_flag;
         } else {
             picture_structure = PICT_FRAME;
-            h->mb_aff_frame      = h->sps.mb_aff;
+            mb_aff_frame      = h->sps.mb_aff;
         }
     }
     if (h->current_slice) {
         if (last_pic_structure != picture_structure ||
-            last_pic_droppable != droppable) {
+            last_pic_droppable != droppable ||
+            last_mb_aff_frame  != mb_aff_frame) {
             av_log(h->avctx, AV_LOG_ERROR,
                    "Changing field mode (%d -> %d) between slices is not allowed\n",
                    last_pic_structure, h->picture_structure);
@@ -1472,6 +1475,7 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
     h->picture_structure = picture_structure;
     h->droppable         = droppable;
     h->frame_num         = frame_num;
+    h->mb_aff_frame      = mb_aff_frame;
     sl->mb_field_decoding_flag = picture_structure != PICT_FRAME;
 
     if (h->current_slice == 0) {