fix peer ae.response bug

If the peer return a high term, we should not update its previous index.
The previous index might larger than the index of this "leader" (it is not leader any more), since the new leader might have successfully append logs to that peer.

If we update the index, panic will happen:
https://github.com/goraft/raft/blob/master/log.go#L254 

I think the panic is correct.