Check for null test server in BookmarkTest#teardown

If the setup fails for some reason, the test server may be null. Check
for null so that the test failure reason shows the startup error.

BUG=1062642

Change-Id: I2a7710da86f94ae50420f010c4ec74a8070381f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2108602Reviewed-by: Lijin Shen <lazzzis@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751387}

chrome/android/javatests/src/org/chromium/chrome/browser/bookmarks/BookmarkTest.java

@@ -157,7 +157,7 @@ public class BookmarkTest {
 
     @After
     public void tearDown() {
-        mTestServer.stopAndDestroyServer();
+        if (mTestServer != null) mTestServer.stopAndDestroyServer();
     }
 
     @AfterClass