session: fix session table retrieval

Change-Id: If37d644a26132c42a04927f612eec5e891066451
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/session/session_table.c b/src/vnet/session/session_table.c
index 5d160a4..47d1909 100644
--- a/src/vnet/session/session_table.c
+++ b/src/vnet/session/session_table.c
@@ -45,9 +45,9 @@
 session_table_t *
 session_table_get (u32 table_index)
 {
-  if (vec_len (lookup_tables) <= table_index)
+  if (pool_is_free_index (lookup_tables, table_index))
     return 0;
-  return vec_elt_at_index (lookup_tables, table_index);
+  return pool_elt_at_index (lookup_tables, table_index);
 }
 
 #define foreach_hash_table_parameter            \