title: Nullpointer exception when trying to receive UDP multicast
body: <p>i'm puzzeled after a few attempts to get a simple UDP multicast receiver working.
  After my own code did not work as expected i tried out the exact example posted in the vertx documentation:</p>
  
  <pre><code>DatagramSocket socket = vertx.createDatagramSocket(new DatagramSocketOptions());
  socket.listen(1234, "0.0.0.0", asyncResult -&gt; {
    if (asyncResult.succeeded()) {
      socket.handler(packet -&gt; {
        // Do something with the packet
      });
  
      // join the multicast group
      socket.listenMulticastGroup("230.0.0.1", asyncResult2 -&gt; {
          System.out.println("Listen succeeded? " + asyncResult2.succeeded());
      });
    } else {
      System.out.println("Listen failed" + asyncResult.cause());
    }
  });
  </code></pre>
  
  <p>After executing, i got the following exception:</p>
  
  <pre><code>java.lang.NullPointerException: networkInterface
      at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:409)
      at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:368)
      at io.netty.channel.socket.nio.NioDatagramChannel.joinGroup(NioDatagramChannel.java:362)
      at io.vertx.core.datagram.impl.DatagramSocketImpl.listenMulticastGroup(DatagramSocketImpl.java:90)
  </code></pre>
  
  <p>I can get it to work when providing the right IP address of the network interface (e.g. 192.168.178.52) instead of 0.0.0.0. However, this means iterating over all network interfaces and adding a socket per interface.</p>
  
  <p>Any ideas? Thank you!</p>