Check name isn't NULL in krb5_parse_name_flags

There is a potential for NULL pointer dereference in
krb5_parse_name_flags in calls of static functions allocate_princ
and parse_name_into_princ. There is at least one code path, that
leads to krb5_parse_name_flag being called with name == NULL:
when out of memory, build_name_with_realm called from add_admin_princ
returns NULL for fullname, which is then passed as name through
krb5_parse_name to krb5_parse_name_flags.

Return error when krb5_parse_name_flag is called with name == NULL.