avformat/mux: fix flush_packets flag with flushing buffers

Found-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -632,7 +632,7 @@ int av_write_frame(AVFormatContext *s, AVPacket *pkt)
     if (!pkt) {
         if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
             ret = s->oformat->write_packet(s, NULL);
-            if (s->flush_packets && s->pb && s->pb->error >= 0)
+            if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
                 avio_flush(s->pb);
             if (ret >= 0 && s->pb && s->pb->error < 0)
                 ret = s->pb->error;