avformat/matroskadec: do not trust the default duration to be the real 1/timebase if its less than 5fps

Fixes Ticket3980

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

libavformat/matroskadec.c

@@ -1920,7 +1920,8 @@ static int matroska_parse_tracks(AVFormatContext *s)
                 av_reduce(&st->avg_frame_rate.num, &st->avg_frame_rate.den,
                           1000000000, track->default_duration, 30000);
 #if FF_API_R_FRAME_RATE
-                if (st->avg_frame_rate.num < st->avg_frame_rate.den * 1000L)
+                if (   st->avg_frame_rate.num < st->avg_frame_rate.den * 1000L
+                    && st->avg_frame_rate.num > st->avg_frame_rate.den * 5L)
                     st->r_frame_rate = st->avg_frame_rate;
 #endif
             }