Fix issue #572 for MQTT V3

Author: Ian Craggs

org.eclipse.paho.client.mqttv3/src/main/java/org/eclipse/paho/client/mqttv3/internal/NetworkModuleService.java

@@ -1,4 +1,6 @@
 /*
+ * Copyright (c) 2009, 2019 IBM Corp.
+ * 
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * and Eclipse Distribution License v1.0 which accompany this distribution.
@@ -53,7 +55,7 @@ private NetworkModuleService() {
 	 * @throws IllegalArgumentException is case the URI is invalid or there is no {@link NetworkModule} installed for
 	 * the URI scheme
 	 */
-	public static void validateURI(String brokerUri) throws IllegalArgumentException {
+	public synchronized static void validateURI(String brokerUri) throws IllegalArgumentException {
 		try {
 			URI uri = new URI(brokerUri);
 			String scheme = uri.getScheme();