ipa_range_check: Use special attributes to determine presence of RID bases
    
    The slapi_entry_attr_get_ulong which is used to get value of the RID base
    attributes returns 0 in case the attribute is not set at all. We need
    to distinguish this situation from the situation where RID base attributes
    are present, but deliberately set to 0.
    
    Otherwise this can cause false negative results of checks in the range_check
    plugin.
    
    Part of: https://fedorahosted.org/freeipa/ticket/4137
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>