Ticket 408 - create a normalized dn cache

covscan Defect type: FORWARD_NULL
15. 389-ds-base-1.3.1.6/ldap/servers/slapd/dn.c:3036:var_deref_op -
    Dereferencing null pointer "node".
Introduced by commit 1d6dd39fb8b0ef8eb42ec9ef8c3d325e27a3d3c1
Fix: Check if "node" is null or not before referencing.
Note: Found second FORWARD_NULL in addition to
      commit 5ac08038a05877b4240755801debd5e5a3a94c6f,

ldap/servers/slapd/dn.c

@@ -3059,7 +3059,7 @@ ndn_cache_free()
     }
 
     node = ndn_cache->tail;
-    while(ndn_cache->cache_count){
+    while(node && ndn_cache->cache_count){
         flush_node = node;
         /* update the lru */
         next = node->prev;