Use global nullifier list in z_listnullifiers

src/wallet/rpcwallet.cpp

@@ -3764,7 +3764,9 @@ UniValue z_listnullifiers(const UniValue& params, bool fHelp, const CPubKey& myp
 
     LOCK2(cs_main, pwalletMain->cs_wallet);
     UniValue ret(UniValue::VARR);
-    for (auto nullifier: mempool.getNullifiers()) {
+    //global set is very large!
+    CNullifiersMap nullifiers = pcoinsTip->getNullifiers();
+    for (auto nullifier : nullifiers) {
         ret.push_back(nullifier.first.GetHex());
     }
     return ret;