avdevice/lavfi: Don't set pkt->size to the value it already has

av_new_packet() already sets the size. And if the packet is not
allocated by av_new_packet() (which seems to be impossible atm), both
pkt->size as well as size are 0, so setting it again is unnecessary in
this scenario, too.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavdevice/lavfi.c

@@ -474,7 +474,6 @@ static int lavfi_read_packet(AVFormatContext *avctx, AVPacket *pkt)
     pkt->stream_index = stream_idx;
     pkt->pts = frame->pts;
     pkt->pos = frame->pkt_pos;
-    pkt->size = size;
     av_frame_unref(frame);
     return size;
 }