ipa-kdb: fix error handling of is_master_host()
    
    Adding proper error handling to the is_master_host() function to allow
    it to make the difference between the absence of a master host object
    and a connection failure. This will keep the krb5kdc daemon from
    continuing to run with a NULL LDAP context.
    
    Fixes: https://pagure.io/freeipa/issue/9422
    
    Signed-off-by: Julien Rische <jrische@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>