core/reloader: null generation ref in reloadables on destruction

On the post-reload reloadable initialzation path, a timer is used to
delay reload(). This change fixes a UAF when switching generations
while that timer is running.

src/core/reload.hpp

@@ -71,6 +71,7 @@ public:
 
 private slots:
 	void onReloadFinished();
+	void onGenerationDestroyed();
 
 protected:
 	// Called unconditionally in the reload phase, with nullptr if no source could be determined.