[bugfix] Avoid nil ptr if maintenance router can't be started (#3919)

internal/cache/cache.go

@@ -144,9 +144,13 @@ func (c *Caches) Start() error {
 func (c *Caches) Stop() {
 	log.Infof(nil, "stop: %p", c)
 
+	if c.Webfinger != nil {
 		_ = c.Webfinger.Stop()
+	}
+	if c.StatusesFilterableFields != nil {
 		_ = c.StatusesFilterableFields.Stop()
 	}
+}
 
 // Sweep will sweep all the available caches to ensure none
 // are above threshold percent full to their total capacity.