Issue description:

When using LDAPObject.search_ext and passing iterables via the attrlist argument the call can fail with a segmentation fault or other types of memory errors.

Steps to reproduce:

With a locally running slapd:

import ldap
l = ldap.initialize("ldap://localhost")
l.simple_bind_s("cn=admin,dc=nodomain","admin")
l.search_s("o=My Organisation, c=AU", ldap.SCOPE_SUBTREE, "objectclass=*", attrlist={"1":2})

This may result in

Segmentation fault (core dumped)

or

free(): invalid pointer
Aborted (core dumped)

or another memory related program abortion.

Operating system:

Distributor ID:	Ubuntu
Description:	Ubuntu 23.04
Release:	23.04
Codename:	lunar

bookworm/sid

Python version:

Tested with 3.7, 3.10, 3.11

python-ldap version:

3.4.4 and latest (ac5d051)

See #555