tl_matroska: fix use-after-free with --cache

If cache was enabled, using ordered chapters could easily crash. The
reason is that enable_cache() reopens the demuxer and closes the old
one. The code after that (reading m->ordered_chapters etc.) then
accessed freed data.

This commit also avoids enabling cache for files which are not used
(which would make opening much slower).
demux_mkv: fill ordered chapters info only if it's present

This was probably not a real problem. But it's not entirely clear
whether this could actually happen or not, so it's better to be
defensive. The code is now also somewhat easier to understand.
demux_mkv: cosmetics: add redundant braces for consistent style

Leaving these braces away just because the syntax allows them is really
obnoxious. It removes the visual cues which help understanding the code
at the first look.

For the record,

  if (cond)
      something();

is ok, as long as there's no else branch, and the if body is one
physical line. But everything else should have braces.
demux_mkv: use standard C default initialization syntax

gcc and clang happen to allow {} to default-initialize a struct, but
strictly speaking, C99 requires at least {0}. In one case, we use {{0}},
but that's only because gcc as well as clang are too damn stupid not
to warn about {0}, which is a perfectly valid construct in this case.

(Sure is funny, don't warn about the non-standard case, but warn about
another standard conform case.)