fix: potential panic in postjoin

No assumptions should be made on `traft::node::Status.leader_id`. It can
be set to `None` at any time due to a re-election.

src/main.rs

@@ -953,7 +953,7 @@ fn postjoin(args: &args::Run, storage: Storage) {
             .with_target_grade(TargetGrade::Online)
             .with_failure_domain(args.failure_domain());
 
-        let leader_id = node.status().leader_id.expect("leader_id deinitialized");
+        let leader_id = unwrap_some_or!(node.status().leader_id, continue);
         let leader = storage.peers.get(&leader_id).unwrap();
 
         // It's necessary to call `proc_update_peer` remotely on a