heimdal: Resolve hdb_free_entry() SIGSEGV/SIGILL

When the client sends kadmind a create principal (kadm_create) request
kadm_s_create_principal() returns an error before zeroing out ent (an
hdb entry structure wrapper -- hdb_entry_ex), resulting in a NULL
reference.

Fix obtained from upstream commit 35ea4955a.

PR: 268059
Reported by: Robert Morris <rtm@lcs.mit.edu>
Obtained from: Heimdal commit 35ea4955a
MFC after: 3 days