AD: Continue if sssd failes to check extra members
    
    Reported by scan-build
    
        for (mi = 0; group_only[mi]; mi++) {
                     ^~~~~~~~~~
    warning: Array access (from variable 'group_only') results in a null pointer
    dereference
    
    It can happend if function ad_group_extra_members fails (ret != EOK)
    
    Reviewed-by: Simo Sorce <simo@redhat.com>
    (cherry picked from commit bad65473c4c28ecbf2b6bd374a7ae2d634d57d8d)