fixed error changing setting in mDNS

lib/mdns.js

@@ -187,9 +187,10 @@ module.exports.create = function (deps, config) {
         promise = stop().then(start);
       } else {
         // Can't check membership, so just add the current broadcast address to make sure
-        // it's set. Hopefully nothing bad happens by adding multiple addresses or adding
-        // the same address multiple times.
-        socket.addMembership(config.mdns.broadcast);
+        // it's set. If it's already set it will throw an exception (at least on linux).
+        try {
+          socket.addMembership(config.mdns.broadcast);
+        } catch (e) {}
         promise = Promise.resolve();
       }
     }