libwbclient: initialize some return values
    
    Some callers of libwbclient functions expects the return values are
    initialized even it the functions returns an error. This patch adds some
    initializations to meet this requirement.
    
    Resolves https://fedorahosted.org/sssd/ticket/2537
    
    Reviewed-by: Pavel Reichl <preichl@redhat.com>