nfsd: fix NULL pointer dereference in cld_pipe_downcall

If we find that "cup" is NULL in this case, then we obviously don't
want to dereference it. What we really want to print in this case
is the xid that we copied off earlier.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>