do not base frame rate guess on just one duration

Originally committed as revision 7383 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -1959,7 +1959,7 @@ int av_find_stream_info(AVFormatContext *ic)
             if(st->codec->codec_id == CODEC_ID_RAWVIDEO && !st->codec->codec_tag && !st->codec->bits_per_sample)
                 st->codec->codec_tag= avcodec_pix_fmt_to_codec_tag(st->codec->pix_fmt);
 
-            if(duration_count[i]
+            if(duration_count[i]>1
                && (st->codec->time_base.num*101LL <= st->codec->time_base.den || st->codec->codec_id == CODEC_ID_MPEG2VIDEO) /*&&
                //FIXME we should not special case mpeg2, but this needs testing with non mpeg2 ...
                st->time_base.num*duration_sum[i]/duration_count[i]*101LL > st->time_base.den*/){