Check for selfinclusion of assets (closes #1032)

Make sure that OpenSpace doesn't crash when bailing out early

include/openspace/scene/asset.h

@@ -61,8 +61,6 @@ public:
      */
     Asset(AssetLoader* loader, SynchronizationWatcher* watcher, std::string assetPath);
 
-    ~Asset() = default;
-
     std::string id() const;
     const std::string& assetFilePath() const;
     bool hasAssetFile() const;