If the constructor moves from an ExportedSong instance which refers to
somebody else's "Tag" instance, the newly constructed instance will
instead refer to its own empty "tag_buffer" field. This broke
SimpleDatabase::GetSong(), i.e. all songs on the queue restored from
the state file or added using the "addid" command.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/1089
After commit 1afa33c3c7, an old bug was revealed:
SimpleDatabase::GetSong() constructs an ExportedSong instance by
moving the return value of Song::Export(), which causes the
LightSong::tag field to be dangling on the moved-from
ExportedSong::tag_buffer. This broke tags from CUE sheets.
Closes https://github.com/MusicPlayerDaemon/MPD/issues/1070
