Null-terminate components of parsed principals

Null-terminate components of parsed principals

The rewritten krb5_parse_name didn't null-terminate components or
realms of principals, while the old one did.  Fix the new one to do so
as well.

This means KRB5_PRINCIPAL_PARSE_IGNORE_REALM allocates one byte for
the realm instead of leaving it as empty_data(), so we need to free
the realm in build_in_tkt_name() before copying in the client realm.