avformat/matroskadec: Check duration

Fixes: -nan is outside the range of representable values of type 'long'
Fixes: 44614/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-6216204841254912

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 36680078)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/matroskadec.c

@@ -2186,6 +2186,8 @@ static int matroska_read_header(AVFormatContext *s)
 
     if (!matroska->time_scale)
         matroska->time_scale = 1000000;
+    if (isnan(matroska->duration))
+        matroska->duration = 0;
     if (matroska->duration)
         matroska->ctx->duration = matroska->duration * matroska->time_scale *
                                   1000 / AV_TIME_BASE;