Remove an unnecessary NULL-check

Remove an unnecessary NULL-check

In the ns__client_put_cb() callback function the 'client->manager'
pointer is guaranteed to be non-NULL, because in ns__client_request(),
before setting up the callback, the ns__client_setup() function is
called for the 'client', which makes sure that 'client->manager' is set.

Removing the NULL-check resolves the following static analyzer warning:

    /lib/ns/client.c: 1675 in ns__client_put_cb()
    1669     		dns_message_puttemprdataset(client->message, &client->opt);
    1670     	}
    1671     	client_extendederror_reset(client);
    1672
    1673     	dns_message_detach(&client->message);
    1674
    >>>     CID 465168:  Null pointer dereferences  (REVERSE_INULL)
    >>>     Null-checking "client->manager" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
    1675     	if (client->manager != NULL) {
    1676     		ns_clientmgr_detach(&client->manager);
    1677     	}
    1678
    1679     	/*
    1680     	 * Detaching the task must be done after unlinking from