cache_req: fix initgroups by name
    
    If overriden name was provided we stole already freed value.
    Name is attached to "user" talloc context which we freed before
    stealing the value. This caused crash in SSSD.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>