service: fix the mutex deadlock at shutdown in service_saver() thread fcn

src/service.c

@@ -1313,6 +1313,8 @@ service_saver(void *aux)
     pthread_mutex_unlock(&global_lock);
     pthread_mutex_lock(&pending_save_mutex);
   }
+
+  pthread_mutex_unlock(&pending_save_mutex);
   return NULL;
 }