dhcp: Only send client identifier if identity_lease is enabled

The client identifier serves as unique identifier just like a unique MAC
address would, so even with identity_leases disabled some DHCP servers
might assign unique leases per identity.

src/libcharon/plugins/dhcp/dhcp_socket.c

@@ -268,7 +268,8 @@ static int prepare_dhcp(private_dhcp_socket_t *this,
 		remaining -= sizeof(dhcp_option_t) + option->len;
 	}
 
-	if (remaining >= sizeof(dhcp_option_t) + 2)
+	if (this->identity_lease &&
+		remaining >= sizeof(dhcp_option_t) + 2)
 	{
 		option = (dhcp_option_t*)&dhcp->options[optlen];
 		option->type = DHCP_CLIENT_ID;