Ticket 48665 - Prevent sefault in ldbm_instance_modify_config_entry
Bug Description: python ldap sends a mod_delete then a mod_add to the server.
This causes a segfault in ldbm_instance_modify_config_entry which expects the
value of the delete operation to exist.
Fix Description: If there is no attribute in the delete operation, this now
passes a NULL to ldbm_config_set, which then rejects the op as NO_SUCH_ATTRIBUTE
There is an attached test case, which also checks that mod_replace is still
functional.
https://fedorahosted.org/389/ticket/48665
Author: wibrown
Review by: nhosoi (Thanks!)
(cherry picked from commit 9134b006948d7d8eeb926655640b7f068fc6724d)