db/simple: fix ExportedSong move constructor for non-owning sources
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
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