matroska: do not set invalid default duration if frame rate is zero

If a video track specifies a zero frame rate (invalid but occurs),
this results in a division by zero and subsequent undefined conversion
to integer.  Setting the default duration from the frame rate only
if the latter is greater than zero avoids such problems.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/matroskadec.c

@@ -1342,7 +1342,7 @@ static int matroska_read_header(AVFormatContext *s)
             continue;
 
         if (track->type == MATROSKA_TRACK_TYPE_VIDEO) {
-            if (!track->default_duration)
+            if (!track->default_duration && track->video.frame_rate > 0)
                 track->default_duration = 1000000000/track->video.frame_rate;
             if (!track->video.display_width)
                 track->video.display_width = track->video.pixel_width;