Fixed a bug where lack of IPv6 destination = fatal

There was a problem where trying to bind .v4 and .v6 listeners would set them
to None if there was nothing to bind (if, say, you weren't binding an IPv6
listener).  Later, the code then would try to call a member function of the
listener.  The member function would not do anything if there was no listener,
but trying to dereference None yielded the broken behavior.

client.py

@@ -681,11 +681,15 @@ def main(listenip_v6, listenip_v4,
         tcp_listener.setsockopt(socket.SOL_IP, IP_TRANSPARENT, 1)
         if udp_listener:
             udp_listener.setsockopt(socket.SOL_IP, IP_TRANSPARENT, 1)
+            if udp_listener.v4 is not None:
                 udp_listener.v4.setsockopt(socket.SOL_IP, IP_RECVORIGDSTADDR, 1)
+            if udp_listener.v6 is not None:
                 udp_listener.v6.setsockopt(SOL_IPV6, IPV6_RECVORIGDSTADDR, 1)
         if dns_listener:
             dns_listener.setsockopt(socket.SOL_IP, IP_TRANSPARENT, 1)
+            if dns_listener.v4 is not None:
                 dns_listener.v4.setsockopt(socket.SOL_IP, IP_RECVORIGDSTADDR, 1)
+            if dns_listener.v6 is not None:
                 dns_listener.v6.setsockopt(SOL_IPV6, IPV6_RECVORIGDSTADDR, 1)
 
     try: