ipadb_mspac_get_trusted_domains: NULL ptr deref

Fix potential NULL pointer deref in ipadb_mspac_get_trusted_domains().
In theory, dn could be empty and rdn NULL. The man page for ldap_str2dn()
does not guarantee that it returns a non-empty result.

See: https://pagure.io/freeipa/issue/7738
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>

daemons/ipa-kdb/ipa_kdb_mspac.c

@@ -2586,6 +2586,12 @@ krb5_error_code ipadb_mspac_get_trusted_domains(struct ipadb_context *ipactx)
         }
 
         /* We should have a single AVA in the domain RDN */
+        if (rdn == NULL) {
+            ldap_dnfree(dn);
+            ret = EINVAL;
+            goto done;
+        }
+
         t[n].parent_name = strndup(rdn[0]->la_value.bv_val, rdn[0]->la_value.bv_len);
 
         ldap_dnfree(dn);