demux_mkv: store streams sequentially in demuxer->[avs]_streams

demux_mkv: store streams sequentially in demuxer->[avs]_streams

demux_mkv used the Matroska TrackNumber as the array offset in demuxer
stream lists. The TrackNumber entry stored in the file can be an
arbitrary 64-bit value, and some of the code could try reading from
the arrays with that offset, causing a crash if the file had insane
values. Fill the arrays sequentially instead. Also add some checks to
make the handling of too high stream counts more robust.