Avoid passing null for asprintf strings

Avoid passing null for asprintf strings

It is undefined behavior to pass null to a printf function for a %.*s
substitution, even if the accompanying length is zero.  OpenBSD
generates syslog warnings from libc when it sees a null pointer in a
string substitution (reported by Nathanael Rensen).
krb5_sname_to_principal() passes a null pointer in the usual case
where there is no port trailer.  Address this case and others where we
use asprintf() with %.*s substitutions and might pass null, either by
avoiding the use of asprintf() or by ensuring that the pointer isn't
null.

ticket: 9047 (new)