Validate MQTT In topics

Fixes #792

nodes/core/io/10-mqtt.html

@@ -1,5 +1,5 @@
 <!--
-  Copyright 2013,2015 IBM Corp.
+  Copyright 2013, 2016 IBM Corp.
   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
@@ -37,7 +37,7 @@
         category: 'input',
         defaults: {
             name: {value:""},
-            topic: {value:"",required:true},
+            topic: {value:"",required:true,validate: RED.validators.regex(/^(#$|(\+|[^+#]*)(\/(\+|[^+#]*))*(\/(\+|#|[^+#]*))?$)/)},
             broker: {type:"mqtt-broker", required:true}
         },
         color:"#d8bfd8",