demux_mkv: discard broken index

Add a simplistic heuristic for detecting broken indexes. This includes
indexes with very few elements (apparently libavformat sometimes writes
such indexes, or used to), and indexes with broken timestamps.

The latter was apparently produced by very old HandBrake versions:

| + Muxing application: libmkv 0.6.1.2
| + Writing application: HandBrake 0.9.1

These broken files seem to be common enough that libavformat added a
workaround for them in 2008 (and maybe again in 2015). Apparently all
timestamps are multiplied with the file's tc_scale twice, and FFmpeg
attempts to fix them. We should throw away the whole thing.

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