Check for asprintf failure in kdb5_util create

In add_admin_princ, remove build_name_with_realm and call asprintf
directly instead.  Check for asprintf failure to avoid passing an
undefined pointer to krb5_parse_name.

[ghudson@mit.edu: rewrite commit message]

ticket: 7902 (new)
target_version: 1.12.2
tags: pullup