lavf: flush the output AVIOContext in av_write_trailer().

This is consistent with stdio and is what we want to do in all cases.

Fixes a bug in the voc muxer which didn't flush in write_trailer()
previously. This is the cause of the change in the test results.

libavformat/ffmenc.c

@@ -226,15 +226,12 @@ static int ffm_write_packet(AVFormatContext *s, AVPacket *pkt)
 
 static int ffm_write_trailer(AVFormatContext *s)
 {
-    AVIOContext *pb = s->pb;
     FFMContext *ffm = s->priv_data;
 
     /* flush packets */
     if (ffm->packet_ptr > ffm->packet)
         flush_packet(s);
 
-    avio_flush(pb);
-
     return 0;
 }