movenc: Ensure no separate moof written for empty track

track->mdat_buf can be not NULL while the track is still empty if the
last packet write failed.

Signed-off-by: Hu Weiwen <sehuww@mail.scut.edu.cn>
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -5422,7 +5422,7 @@ static int mov_flush_fragment(AVFormatContext *s, int force)
             duration = track->start_dts + track->track_duration -
                        track->cluster[0].dts;
         if (mov->flags & FF_MOV_FLAG_SEPARATE_MOOF) {
-            if (!track->mdat_buf)
+            if (!track->entry)
                 continue;
             mdat_size = avio_tell(track->mdat_buf);
             moof_tracks = i;