8237890: DatagramPacket::getSocketAddress doesn't specify what happens if address or port are not set

This fix changes the default port of a DatagramPacket from -1 to 0, which changes the behaviour of calling getSocketAddress with no port set from throwing an IAE to returning an InetSocketAddress representing any local address with port 0.

Reviewed-by: alanb, chegar, dfuchs

src/java.base/share/classes/sun/nio/ch/DatagramSocketAdaptor.java

@@ -209,7 +209,6 @@ public class DatagramSocketAdaptor
                     p.setPort(remote.getPort());
                     target = remote;
                 } else {
-                    // throws IllegalArgumentException if port not set
                     target = (InetSocketAddress) p.getSocketAddress();
                 }
             }