HADOOP-15294. TestUGILoginFromKeytab fails on Java9

Signed-off-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/UserGroupInformation.java

@@ -1885,7 +1885,10 @@ public class UserGroupInformation {
     @Override
     public void logout() throws LoginException {
       synchronized(getSubjectLock()) {
-        super.logout();
+        if (this.getSubject() != null
+            && !this.getSubject().getPrivateCredentials().isEmpty()) {
+          super.logout();
+        }
       }
     }
   }

hadoop-common-project/hadoop-minikdc/src/test/java/org/apache/hadoop/minikdc/TestMiniKdc.java

@@ -166,7 +166,7 @@ public class TestMiniKdc extends KerberosSecurityTestcase {
 
     } finally {
       if (loginContext != null && loginContext.getSubject() != null
-          && !loginContext.getSubject().getPrincipals().isEmpty()) {
+          && !loginContext.getSubject().getPrivateCredentials().isEmpty()) {
         loginContext.logout();
       }
     }