player: ordered chapters: filter missing chapters

Ever since someone (not me) added some Matroska features which nobody
ever uses (ordered edition or some bullshit), I haven't had a fucking
clue what the fuck is going on in this fucking file. (Still agreed to
it, so it's my fault.)

mplayer2 handled missing chapters correctly (and I suppose in a somewhat
clean/simple manner), but the changed code doesn't. Since I can't even
follow this code because it's so arcanely complicated, just add a hack
that has the same effect.
player: ordered chapters: bullshit reduction

Nobody cares about these redundant messages, they make the code even
more complicated than it has to be, and also they're annoying.
player: ordered chapters: cosmetics

Drop some meaningless comments, fix codingstyle, fix some formatting
victims of the mp_msg refactoring, replace some memcpys with
assignments.