avpacket: properly reset data/size in av_packet_move_ref()

It currently just calls av_init_packet(), which does not touch those
fields.

libavcodec/avpacket.c

@@ -398,6 +398,8 @@ void av_packet_move_ref(AVPacket *dst, AVPacket *src)
 {
     *dst = *src;
     av_init_packet(src);
+    src->data = NULL;
+    src->size = 0;
 }
 
 void av_packet_rescale_ts(AVPacket *pkt, AVRational src_tb, AVRational dst_tb)