vd_lavc: signal packet consumed in drop-all case

This is just a very special code path. This probably got stuck, now that
the previous commit returned the EAGAIN properly. Untested.

video/decode/vd_lavc.c

@@ -977,7 +977,7 @@ static int do_send_packet(struct mp_filter *vd, struct demux_packet *pkt)
         return AVERROR_UNKNOWN;
 
     if (avctx->skip_frame == AVDISCARD_ALL)
-        return AVERROR(EAGAIN);
+        return 0;
 
     AVPacket avpkt;
     mp_set_av_packet(&avpkt, pkt, &ctx->codec_timebase);