DatabasePlugin: add method ReturnSong()

Allow the plugin to allocate the GetSong() return value.

src/db/ProxyDatabasePlugin.cxx

@@ -56,6 +56,8 @@ public:
 	virtual void Close() override;
 	virtual struct song *GetSong(const char *uri_utf8,
 				     GError **error_r) const override;
+	virtual void ReturnSong(struct song *song) const;
+
 	virtual bool Visit(const DatabaseSelection &selection,
 			   VisitDirectory visit_directory,
 			   VisitSong visit_song,
@@ -191,6 +193,16 @@ ProxyDatabase::GetSong(const char *uri, GError **error_r) const
 	return nullptr;
 }
 
+void
+ProxyDatabase::ReturnSong(struct song *song) const
+{
+	assert(song != nullptr);
+	assert(song_in_database(song));
+	assert(song_is_detached(song));
+
+	song_free(song);
+}
+
 static bool
 Visit(struct mpd_connection *connection, const char *uri,
       bool recursive, VisitDirectory visit_directory, VisitSong visit_song,