cache_req: do not set ncache if dp request fails
    
    We will only remember entry in the negative cache if the data provider
    requests succeeded because only then we can be sure that the entry
    does not exist.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>