Do not process zero or hashed_null_node pair trie roots.

Log the case when these are received from other nodes.
#807

mm2src/lp_ordermatch.rs

@@ -326,7 +326,9 @@ fn remove_and_purge_pubkey_pair_orders(orderbook: &mut Orderbook, pubkey: &str,
     }
 
     if orderbook.memory_db.remove_and_purge(&pair_root, EMPTY_PREFIX).is_none() {
-        log::warn!("Warning: couldn't find {:?} hash root in memory_db", pair_root);
+        if pair_root != H64::default() && pair_root != hashed_null_node::<Layout>() {
+            log::warn!("Warning: couldn't find {:?} hash root in memory_db", pair_root);
+        }
     }
 }
 
@@ -1999,6 +2001,15 @@ impl Orderbook {
                 continue;
             }
 
+            if trie_root == H64::default() || trie_root == hashed_null_node::<Layout>() {
+                log::warn!(
+                    "Received zero or hashed_null_node pair {} trie root from pub {}",
+                    alb_pair,
+                    from_pubkey
+                );
+
+                continue;
+            }
             let actual_trie_root = order_pair_root_mut(&mut pubkey_state.trie_roots, &alb_pair);
             if *actual_trie_root != trie_root {
                 trie_roots_to_request.insert(alb_pair, trie_root);