always write footer partition according to specs

Originally committed as revision 15100 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mxfenc.c

@@ -857,11 +857,10 @@ static int mxf_write_footer(AVFormatContext *s)
     ByteIOContext *pb = s->pb;
     int64_t byte_position= url_ftell(pb);
 
-    if (!url_is_streamed(s->pb)) {
+    mxf_write_partition(s, byte_position, 0, footer_partition_key);
-        mxf_write_partition(s, byte_position, 0, footer_partition_key);
+    put_flush_packet(pb);
-        put_flush_packet(pb);
+    if (!url_is_streamed(s->pb))
         mxf_update_header_partition(s, byte_position);
-    }
     mxf_free(s);
     return 0;
 }