lavf: allow BSFs to drop packets.

If pkt->size == 0 && pkt->side_data_elems == 0 after bsf->filter()
returns, the packet is considered dropped.

ffmpeg.c

@@ -694,6 +694,8 @@ static void write_frame(AVFormatContext *s, AVPacket *pkt, OutputStream *ost)
         if (exit_on_error)
             exit_program(1);
     }
+    if (pkt->size == 0 && pkt->side_data_elems == 0)
+        return;
 
     if (!(s->oformat->flags & AVFMT_NOTIMESTAMPS)) {
         if (pkt->dts != AV_NOPTS_VALUE &&