DefaultSocks5CommandRequest incorrectly rejects SOCKS5 commands with dstPort=0
### Expected behavior
According to [SOCKS 5 spec](https://www.ietf.org/rfc/rfc1928.txt), dstPort = 0 is a valid value in case of UDP ASSOCIATE command:

> If the client is not in possesion of the information at the time of the UDP ASSOCIATE, the client MUST use a port number and address of all zeros.

### Actual behavior
However, constructor of DefaultSocks5CommandRequest rejects this value as invalid:

    if (dstPort <= 0 || dstPort >= 65536) {
            throw new IllegalArgumentException("dstPort: " + dstPort + " (expected: 1~65535)");
    }

### Netty version
4.1.15.Final