avpacket: fix copying side data in av_packet_copy_props()

Side data count is incremented by by calling av_packet_new_side_data()
in the following loop, setting it explicitly results in the resulting
value being twice what it should be.

CC: libav-stable@libav.org

libavcodec/avpacket.c

@@ -325,7 +325,6 @@ int av_packet_copy_props(AVPacket *dst, const AVPacket *src)
     dst->convergence_duration = src->convergence_duration;
     dst->flags                = src->flags;
     dst->stream_index         = src->stream_index;
-    dst->side_data_elems      = src->side_data_elems;
 
     for (i = 0; i < src->side_data_elems; i++) {
          enum AVPacketSideDataType type = src->side_data[i].type;