assert no double close

Author: ywangd

modules/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3ClientsManager.java

@@ -237,6 +237,8 @@ void close() {
                 IOUtils.closeWhileHandlingException(perProjectClientsHolders.values());
             }
             IOUtils.closeWhileHandlingException(clusterClientsHolder);
+        } else {
+            assert false : "attempting to close s3 clients manager multiple times";
         }
     }