INI: Prevent null return_cfg during augment
    
    This fixes the behavior of ini_config_augment so that result_cfg
    will not be null unless out of memory. In particular, when base_cfg
    is non-NULL and a fatal error occurred, result_cfg will now be a
    copy of base_cfg. This reflects existing documentation, as base_cfg
    will be augmented and result_cfg is the result of the merge.
    
    Resolves:
    https://pagure.io/SSSD/ding-libs/issue/2776
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>