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>