[Cronet] Add null-check to avoid crashing case where no tests are run

components/cronet/android/test/javatests/src/org/chromium/net/BrotliTest.java

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.net;
 
 import android.support.test.filters.SmallTest;
 
 import org.chromium.base.test.util.Feature;
 
 /**
  * Simple test for Brotli support.
  */
 public class BrotliTest extends CronetTestBase {
     private CronetEngine mCronetEngine;
 
     @Override
     protected void setUp() throws Exception {
         super.setUp();
-        // Load library first to create MockCertVerifier.
-        System.loadLibrary("cronet_tests");
         assertTrue(Http2TestServer.startHttp2TestServer(
                 getContext(), SERVER_CERT_PEM, SERVER_KEY_PKCS8_PEM));
     }
 
     @Override
     protected void tearDown() throws Exception {
         assertTrue(Http2TestServer.shutdownHttp2TestServer());
-        mCronetEngine.shutdown();
+        if (mCronetEngine != null) {
+            mCronetEngine.shutdown();
+        }
         super.tearDown();
     }
 
     @SmallTest
     @Feature({"Cronet"})
     @OnlyRunNativeCronet
     public void testBrotliAdvertised() throws Exception {
         ExperimentalCronetEngine.Builder builder =
                 new ExperimentalCronetEngine.Builder(getContext());
         builder.enableBrotli(true);
@@ skipping 41 matching lines @@
 
     private TestUrlRequestCallback startAndWaitForComplete(String url) {
         TestUrlRequestCallback callback = new TestUrlRequestCallback();
         UrlRequest.Builder builder =
                 mCronetEngine.newUrlRequestBuilder(url, callback, callback.getExecutor());
         builder.build().start();
         callback.blockForDone();
         return callback;
     }
 }