util/find_uid.c: fixed race condition bug
    
    It was wrong to return EOK from get_uid_from_pid() in case of failed
    open() or fstat() as this leaves `uid` uninitialized and no means
    for caller to detect this situation.
    
    There was no reason to fail get_active_uid_linux() completely in case
    of failed get_uid_from_pid() for one of /proc entries. Function was
    changed to continue with next entry instead.
    
    Resolves: https://pagure.io/SSSD/sssd/issue/2854
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>