Bug 630097 - (cov#15507,15508) NULL dereference in entryrdn code

In entryrdn_compare_dups(), we dereference the a and b parameters
when initializing the elem_a and elem_b variables.  We later
perform NULL checks on both a and b, but a NULL would have
triggered a crash.  We should not dereference a or b until after
the NULL checks are performed.

ldap/servers/slapd/back-ldbm/ldbm_entryrdn.c

@@ -173,8 +173,8 @@ entryrdn_get_noancestorid()
 int
 entryrdn_compare_dups(DB *db, const DBT *a, const DBT *b)
 {   
-    rdn_elem *elem_a = (rdn_elem *)a->data;
-    rdn_elem *elem_b = (rdn_elem *)b->data;
+    rdn_elem *elem_a = NULL;
+    rdn_elem *elem_b = NULL;
     int delta = 0;
 
     if (NULL == a) {
@@ -187,6 +187,9 @@ entryrdn_compare_dups(DB *db, const DBT *a, const DBT *b)
         return 1;
     }
 
+    elem_a = (rdn_elem *)a->data;
+    elem_b = (rdn_elem *)b->data;
+
     delta = strcmp((char *)elem_a->rdn_elem_nrdn_rdn,
                    (char *)elem_b->rdn_elem_nrdn_rdn);