Fix origin handling in dn_to_dnsname() for zone DNs.
    
    This patch fixes case where DN is zone (i.e. DN with single idnsName
    component) and origin is non-NULL.
    
    Function str_to_isc_buffer() was fixed to not truncate last character.
    
    Signed-off-by: Petr Spacek <pspacek@redhat.com>