fellow_log: fix bad assertions in logbuffer_ref()

flush_active() can be called multiple times from logbuffer_flush(), and
so can logbuffer_ref()

src/fellow_log.c

@@ -2543,8 +2543,6 @@ logbuffer_ref(struct fellow_logbuffer *lbuf)
 
 	assert(logbuffer_can(lbuf, LBUF_CAN_REF));
 
-	AZ(todo->magic);
-	assert(todo->where == LOGOFF_INVAL);
 	INIT_OBJ(todo, FELLOW_DISK_LOG_OFF_TODO_MAGIC);
 
 	todo->off = lbuf->active.off;