prefer container time_base for frame duration guess

Originally committed as revision 4190 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -632,17 +632,17 @@ static void compute_frame_duration(int *pnum, int *pden, AVStream *st,
     *pden = 0;
     switch(st->codec.codec_type) {
     case CODEC_TYPE_VIDEO:
-        if(st->codec.time_base.num*1000 <= st->codec.time_base.den){
+        if(st->time_base.num*1000 > st->time_base.den){
             *pnum = st->time_base.num;
             *pden = st->time_base.den;
-        }else{
+        }else if(st->codec.time_base.num*1000 > st->codec.time_base.den){
             *pnum = st->codec.time_base.num;
             *pden = st->codec.time_base.den;
-        }
             if (pc && pc->repeat_pict) {
                 *pden *= 2;
                 *pnum = (*pnum) * (2 + pc->repeat_pict);
             }
+        }
         break;
     case CODEC_TYPE_AUDIO:
         frame_size = get_audio_frame_size(&st->codec, pkt->size);