Fix NPE in NettyWebServer stop

Fixes gh-9542

Author: mbhave

Diff for: spring-boot/src/main/java/org/springframework/boot/web/embedded/netty/NettyWebServer.java

@@ -110,7 +110,9 @@ public void stop() throws WebServerException {
 		if (context != null) {
 			context.dispose();
 		}
-		this.latch.countDown();
+		if (this.latch != null) {
+			this.latch.countDown();
+		}
 	}
 
 	@Override

Diff for: spring-boot/src/test/java/org/springframework/boot/web/embedded/netty/NettyWebServerTests.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.web.embedded.netty;
+
+import org.junit.Test;
+
+/**
+ * Tests for {@link NettyWebServer}.
+ *
+ * @author Madhura Bhave
+ */
+public class NettyWebServerTests {
+
+	private NettyWebServer server;
+
+	@Test
+	public void stopWhenLatchIsNullShouldNotThrowException() throws Exception {
+		this.server = new NettyWebServer(null, null);
+		this.server.stop();
+	}
+}