Fix memory leak with drag & drop on 2D viewport

tools/editor/plugins/canvas_item_editor_plugin.cpp

@@ -3797,6 +3797,7 @@ bool CanvasItemEditorViewport::_create_instance(Node* parent, String& path, cons
 
 	if (editor->get_edited_scene()->get_filename()!="") { // cyclical instancing
 		if (_cyclical_dependency_exists(editor->get_edited_scene()->get_filename(), instanced_scene)) {
+			memdelete(instanced_scene);
 			return false;
 		}
 	}