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

Simplify the plugin by using a trivial relative URI; later,
playlist_check_translate_song() will set the correct full URI.

Fixes -Wreturn-std-move (clang 8).

More for https://github.com/MusicPlayerDaemon/MPD/issues/42