In MatroskaExtractor, if the last cue time exceeds the duration
specified in the header, then we end up generating a negative
duration chunk as the last item in the SeekMap. We should probably
not do this, so drop it instead.
Note: Matroska does have a CueDuration element, but it's not used
in the one problematic file I've found.
PiperOrigin-RevId: 285738418
