db/simple: fix dangling LightSong::tag reference in moved ExportedSong

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

src/song/LightSong.hxx

@@ -88,6 +88,19 @@ struct LightSong {
 	LightSong(const char *_uri, const Tag &_tag) noexcept
 		:uri(_uri), tag(_tag) {}
 
+	/**
+	 * A copy constructor which copies all fields, but only sets
+	 * the tag to a caller-provided reference.  This is used by
+	 * the #ExportedSong move constructor.
+	 */
+	LightSong(const LightSong &src, const Tag &_tag) noexcept
+		:directory(src.directory), uri(src.uri),
+		 real_uri(src.real_uri),
+		 tag(_tag),
+		 mtime(src.mtime),
+		 start_time(src.start_time), end_time(src.end_time),
+		 audio_format(src.audio_format) {}
+
 	gcc_pure
 	std::string GetURI() const noexcept {
 		if (directory == nullptr)