Fix possible Tween leak on exit

(cherry picked from commit e173710963c3316833ac2e2c26ab3830551fd790)

scene/main/scene_tree.cpp

@@ -777,6 +777,12 @@ void SceneTree::finish() {
 		E->get()->release_connections();
 	}
 	timers.clear();
+
+	// Cleanup tweens.
+	for (List<Ref<SceneTreeTween>>::Element *E = tweens.front(); E; E = E->next()) {
+		E->get()->clear();
+	}
+	tweens.clear();
 }
 
 void SceneTree::quit(int p_exit_code) {