Set TTL appropriately in AVL::lookup

It should be AVL_TTL(num_items), which is logarithmic in num_items, not
just num_items itself.

avl.cpp

@@ -989,7 +989,7 @@ bool AVL::lookup(MPCTIO &tio, yield_t &yield, RegXS ptr, RegAS key, Duoram<Node>
 bool AVL::lookup(MPCTIO &tio, yield_t &yield, RegAS key, Node *ret_node) {
     auto A = oram.flat(tio, yield);
     RegBS isDummy;
-    bool found = lookup(tio, yield, root, key, A, num_items, isDummy, ret_node);
+    bool found = lookup(tio, yield, root, key, A, AVL_TTL(num_items), isDummy, ret_node);
     return found;
 }