lavf/utils: fix overestimation of the rational number density.

Fixes Ticket498
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -3016,7 +3016,7 @@ int avformat_write_header(AVFormatContext *s, AVDictionary **options)
                 goto fail;
             }
             if(av_cmp_q(st->sample_aspect_ratio, st->codec->sample_aspect_ratio)
-               && FFABS(av_q2d(st->sample_aspect_ratio) - av_q2d(st->codec->sample_aspect_ratio)) > 0.001
+               && FFABS(av_q2d(st->sample_aspect_ratio) - av_q2d(st->codec->sample_aspect_ratio)) > 0.004*av_q2d(st->sample_aspect_ratio)
             ){
                 av_log(s, AV_LOG_ERROR, "Aspect ratio mismatch between encoder and muxer layer\n");
                 ret = AVERROR(EINVAL);