openldap: fix uninitialized ptr access problem

To be safe, the ldap_pvt_thread_pool_getkey() function should
always set the data parameter.  If this is not done, slapd can
segfault during shutdown.

Signed-off-by: Joe Slater <jslater@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>