Unittest NodeTest.LeaderFailWithWitness is not stable.

NodeTest.LeaderFailWithWitness 测试用例：
https://github.com/baidu/braft/blob/b37c610039aa34d6df2e5bda12f830003561b08b/test/test_node.cpp#L462-L477

这段逻辑应该是 2 个 Replica 和一个 Witness 节点，当选出 leader 后，杀死leader，并用 follower 进行日志提交。
但是在这个 for 循环之前，有可能该 follower 已经成为 leader 了，导致 followers 的个数只有 1 个，使用 nodes[1] 会越界。

@lintanghui

Start with NodeTest.LeaderFailWithWitness in test/test_node.cpp around lines 462-477. Reproduce the unstable election scenario where the follower may already become leader before the loop, then trace how followers and nodes[1] are selected. Done means the test remains valid when leadership changes and no longer risks an out-of-bounds access.

Written by the indexing model from the issue text.