avpacket: reset dst side_data fields in av_packet_copy_props

This effectively copies the side data elements from src instead of
potentially merging them with those already existing in dst.
This by extension also removes the only dependency on existing values
in the dst packet.
Reviewed-by: wm4 <nfxjfg@googlemail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/avpacket.c

@@ -571,6 +571,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
     dst->flags                = src->flags;
     dst->stream_index         = src->stream_index;
 
+    dst->side_data            = NULL;
+    dst->side_data_elems      = 0;
     for (i = 0; i < src->side_data_elems; i++) {
          enum AVPacketSideDataType type = src->side_data[i].type;
          int size          = src->side_data[i].size;