Description of problem:

A device I have only does LDAP auth to Active Directory. Therefore I have configured the PAM passthru plugin and addn plugin to dirsrv. I then setup pam to use pam_radius_auth, but I don't think that is required to reproduce this. If an attempt is made to bind to a dn that doesn't exist, ns-slapd segfaults:

kernel: ns-slapd[26414]: segfault at 0 ip 00007ff0e692a91d sp 00007ff0d12df6c0 error 4 in libpam-passthru-plugin.so[7ff0e6925000+8000]

Version-Release number of selected component (if applicable):

389-ds-base-1.3.8.4-23.el7.x86_64

How reproducible:

100% once minimal configuration made on fresh install of ipa-server with 389-ds-base.

Steps to Reproduce:
1. configure addn plugin per https://www.port389.org/docs/389ds/howto/howto-addn.html
2. configure pam passthru per https://directory.fedoraproject.org/docs/389ds/howto/howto-pam-pass-through.html
3. attempt to ldapsearch -D 'bob' (or any nonexistant user)

Actual results:
ldapsearch fails that server is unavailable

Expected results:
query results.

Additional info:

gdb output -- 

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffd1988700 (LWP 27086)]
0x00007fffe5fd091d in pam_passthru_bindpreop (pb=0x5555572f9980) at ldap/servers/plugins/pam_passthru/pam_ptpreop.c:439
439         if ((method != LDAP_AUTH_SIMPLE) || (*normbinddn == '\0') ||


the non-pam ldap/servers/plugins/passthru/ptpreop.c does this:

   121      normbinddn = slapi_sdn_get_dn(sdn);
   122      if (normbinddn == NULL) {
   123          normbinddn = "";
   124      }

but looks like pam_passthru has no NULL check. I am so far successful using this patch:

[root@kdc1 SOURCES]# cat 0024-Fix-pam-passthrough.patch
diff -ru a/ldap/servers/plugins/pam_passthru/pam_ptpreop.c b/ldap/servers/plugins/pam_passthru/pam_ptpreop.c
--- a/ldap/servers/plugins/pam_passthru/pam_ptpreop.c   2018-06-21 12:55:37.000000000 -0400
+++ b/ldap/servers/plugins/pam_passthru/pam_ptpreop.c   2019-04-17 22:55:25.847280405 -0400
@@ -436,8 +436,8 @@
      * We only handle simple bind requests that include non-NULL binddn and
      * credentials.  Let the Directory Server itself handle everything else.
      */
-    if ((method != LDAP_AUTH_SIMPLE) || (*normbinddn == '\0') ||
-        (creds->bv_len == 0)) {
+    if ((method != LDAP_AUTH_SIMPLE) || (normbinddn == NULL) ||
+        (*normbinddn == '\0') || (creds->bv_len == 0)) {
         slapi_log_err(SLAPI_LOG_PLUGIN, PAM_PASSTHRU_PLUGIN_SUBSYSTEM,
                       "pam_passthru_bindpreop - Not handled (not simple bind or NULL dn/credentials)\n");
         return retcode;