movenc: Automatically flush after writing the initial moov

In most other cases when writing fragmented mp4 files, the output
IO context is flushed after each fragment. Also flush it after
writing the initial moov, to have it behave in the same way.
Signed-off-by: NMartin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -4048,6 +4048,7 @@ static int mov_write_header(AVFormatContext *s)
     if (mov->flags & FF_MOV_FLAG_EMPTY_MOOV &&
         !(mov->flags & FF_MOV_FLAG_DELAY_MOOV)) {
         mov_write_moov_tag(pb, mov, s);
+        avio_flush(pb);
         mov->moov_written = 1;
         if (mov->flags & FF_MOV_FLAG_GLOBAL_SIDX)
             mov->reserved_header_pos = avio_tell(pb);