Prevents 2x stop() calls during IntegrationTest

It seems that NoteDB does not manage gracefully the double
invocations of the GerritServer.stop and throws a NPE.

However, that is really a corner case that never happens in
real life; setting the server instance to null prevents
multiple close() invocations on the same server object.

Change-Id: I34da521830ca4caa935c2968c567cc5f4daca44d

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/AbstractDaemonTest.java

@@ -514,6 +514,7 @@ public abstract class AbstractDaemonTest {
     }
     if (server != commonServer) {
       server.stop();
+      server = null;
     }
   }