Works around assertion failure in the boost::intrusive::set class
because we modified the container between insert_check() and
insert_commit().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/691

`new T[0]` must not be nullptr.

For STL completeness.

release v0.21.17

Closes https://github.com/MusicPlayerDaemon/MPD/issues/685

Prepare for allowing ISO8601 time stamps without delimiters, such as
20191216, and prevent them from being interpreted as numeric UNIX time
stamps.

And allow "Z" suffix after date.

Prepare for adding more flexible parsing.

Move code from Parser.cxx.

This is recommended by the strptime() manpage, because strptime() does
not initialize/set attributes which were not specified in the format
string.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/562