Filip Roséen authored Mar 04, 2016 and Jean-Baptiste Kempf committed Mar 05, 2016

`MkvTree` can be used all across the module, meaning that it better
belongs in `Util.hpp` than with static linkage in `matroska_segment_parse.cpp`.

 - A helper function `MkvTree_va` has been introduced so that one can
   easily call the function from functions that accepts a variadic
   number of arguments (using va_list).

 - The implementation has been optimized so that we do not need to
   allocate storage for the format string on every invocation if it
   would fit inside a fixed length buffer.

 - An error check has been added to signal if we run out of memory.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>