raft.getCluster().getMember(*).memberId() could Result in an NPE

[Xie Dongfang]

Expected behavior
At line 178 of io.atomix.protocols.raft.roles.ActiveRole, the return value of raft.getCluster().getMember(raft.getLastVotedFor()) should be defended by a null check to avoid NullPointerException.

protected VoteResponse handleVote(VoteRequest request) {
    //...
    else {
      log.debug("Rejected {}: already voted for {}", request, raft.getCluster().getMember(raft.getLastVotedFor()).memberId()); // NPE risk
      //...
    }
}

Actual behavior & Steps to reproduce
raft.getCluster().getMember(raft.getLastVotedFor()) could return null when raft's last vote has no corresponding DefaultRaftMember and it's dereferenced unconditionally.

Minimal yet complete reproducer code (or URL to code)

protected VoteResponse handleVote(VoteRequest request) {
    //...
    else {
      DefaultRaftMember member = request, raft.getCluster().getMember(raft.getLastVotedFor());
      log.debug("Rejected {}: already voted for {}", member == null? null:member.memberId()); // NPE risk
      //...
    }
}

Environment

• Atomix: default master
• OS: [e.g. uname -a]
• JVM [e.g. java -version]

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