ZOOKEEPER-3662: Remove NPE Possibility in Follower Class

Author: David Mollitor <dmollitor@apache.org>

Reviewers: Allan Lyu <fangmin@apache.org>, Enrico Olivelli <eolivelli@apache.org>, Norbert Kalmar <nkalmar@apache.org>

Closes #1192 from belugabehr/ZOOKEEPER-3662

zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/Follower.java

@@ -23,6 +23,7 @@ import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.Collections;
 import java.util.Map;
+import java.util.Objects;
 import org.apache.jute.Record;
 import org.apache.zookeeper.ZooDefs.OpCode;
 import org.apache.zookeeper.common.Time;
@@ -48,10 +49,10 @@ public class Follower extends Learner {
 
     ObserverMaster om;
 
-    Follower(QuorumPeer self, FollowerZooKeeperServer zk) {
-        this.self = self;
+    Follower(final QuorumPeer self, final FollowerZooKeeperServer zk) {
+        this.self = Objects.requireNonNull(self);
+        this.fzk = Objects.requireNonNull(zk);
         this.zk = zk;
-        this.fzk = zk;
     }
 
     @Override
@@ -253,14 +254,9 @@ public class Follower extends Learner {
      * @return zxid
      */
     public long getZxid() {
-        try {
-            synchronized (fzk) {
-                return fzk.getZxid();
-            }
-        } catch (NullPointerException e) {
-            LOG.warn("error getting zxid", e);
+        synchronized (fzk) {
+            return fzk.getZxid();
         }
-        return -1;
     }
 
     /**