fix edts for tracks without b frames

Originally committed as revision 5282 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/movenc.c

@@ -946,7 +946,10 @@ static int mov_write_edts_tag(ByteIOContext *pb, MOVTrack *track)
 
     put_be32(pb, av_rescale_rnd(track->trackDuration, globalTimescale, track->timescale, AV_ROUND_UP)); /* duration   ... doesn't seem to effect psp */
 
-    put_be32(pb, track->sampleDuration);
+    if (track->hasBframes)
+        put_be32(pb, track->sampleDuration); /* first pts is 1 */
+    else
+        put_be32(pb, 0);
     put_be32(pb, 0x00010000);
     return 0x24;
 }