mov: Compute max duration among the tracks with a timescale

Prevent an assert.

libavformat/movenc.c

@@ -1640,7 +1640,7 @@ static int mov_write_mvhd_tag(AVIOContext *pb, MOVMuxContext *mov)
     int version;
 
     for (i = 0; i < mov->nb_streams; i++) {
-        if (mov->tracks[i].entry > 0) {
+        if (mov->tracks[i].entry > 0 && mov->tracks[i].timescale) {
             max_track_len_temp = av_rescale_rnd(mov->tracks[i].track_duration,
                                                 MOV_TIMESCALE,
                                                 mov->tracks[i].timescale,