fix: Fixes a bug which prevents the socket to close.

After the delegate is closed, isClosed() would return true,
which would prevent the DatagramSocket instance (super) from
actually closing and releasing its resources.

Author: bgrozev

src/main/java/org/ice4j/socket/DelegatingDatagramSocket.java

@@ -115,6 +115,11 @@ static boolean logNonStun(long numOfPacket)
      */
     private long nbSentPackets = 0;
 
+    /**
+     * Whether this socket has been closed.
+     */
+    private boolean closed = false;
+
     /**
      * Initializes a new <tt>DelegatingDatagramSocket</tt> instance and binds it
      * to any available port on the local host machine.  The socket will be
@@ -282,10 +287,14 @@ public void bind(SocketAddress addr)
     @Override
     public void close()
     {
+        // We want both #delegate and super to actually get closed (and release
+        // the FDs which they hold). But super will not close unless isClosed()
+        // returns false. So we update the #closed flag last.
         if (delegate != null)
             delegate.close();
 
         super.close();
+        closed = true;
     }
 
     /**
@@ -610,7 +619,7 @@ public boolean isBound()
     @Override
     public boolean isClosed()
     {
-        return (delegate == null) ? super.isClosed() : delegate.isClosed();
+        return closed;
     }
 
     /**