h264: split reading the ref list modifications and actually building the ref list

This will allow postponing the reference list construction (and by
consequence some other functions, like frame_start) until the whole
slice header has been parsed.

libavcodec/h264_slice.c

@@ -1347,6 +1347,9 @@ static int h264_slice_header_parse(H264Context *h, H264SliceContext *sl)
            sl->ref_count[1] = sl->ref_count[0] = 0;
            return ret;
        }
+       ret = ff_h264_build_ref_list(h, sl);
+       if (ret < 0)
+           return ret;
     }
 
     if ((pps->weighted_pred && sl->slice_type_nos == AV_PICTURE_TYPE_P) ||