prevent writing empty stss atom

Originally committed as revision 12636 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/movenc.c

@@ -734,7 +734,7 @@ static int mov_write_stbl_tag(ByteIOContext *pb, MOVTrack* track)
     mov_write_stsd_tag(pb, track);
     mov_write_stts_tag(pb, track);
     if (track->enc->codec_type == CODEC_TYPE_VIDEO &&
-        track->hasKeyframes < track->entry)
+        track->hasKeyframes && track->hasKeyframes < track->entry)
         mov_write_stss_tag(pb, track);
     if (track->enc->codec_type == CODEC_TYPE_VIDEO &&
         track->hasBframes)