hdb: Make HDB-as-keytab get function match API

krb5_kt_get_entry() allows a NULL principal to be given ("match all").

The get method of the HDB-as-keytab keytab did not know this, and could
dereference a NULL as a result.

lib/hdb/keytab.c

@@ -190,6 +190,9 @@ hdb_get_entry(krb5_context context,
     HDB *db;
     size_t i;
 
+    if (!principal)
+        return KRB5_KT_NOTFOUND;
+
     memset(&ent, 0, sizeof(ent));
 
     if (dbname == NULL) {