h264: always set redundant_pic_count during slice header parsing

It is always checked in the surrounding code, so this make sure we don't
see a value from an old slice.

libavcodec/h264_slice.c

@@ -1452,6 +1452,7 @@ static int h264_slice_header_parse(H264SliceContext *sl, const H2645NAL *nal,
             sl->delta_poc[1] = get_se_golomb(&sl->gb);
     }
 
+    sl->redundant_pic_count = 0;
     if (pps->redundant_pic_cnt_present)
         sl->redundant_pic_count = get_ue_golomb(&sl->gb);