gui: Fix race in WalletModel::pollBalanceChanged

Poll function was wrongly setting cached height to the current chain height
instead of the chain height at the time of polling.

This bug could cause balances to appear out of date, and was first introduced
bitcoin@a0704a8#diff-2e3836af182cfb375329c3463ffd91f8L117.
Before that commit, there wasn't a problem because cs_main was held during the
poll update.

Currently, the problem should be rare. But if
8937d99 from bitcoin#17954 were merged, the problem
would get worse, because the wrong cachedNumBlocks value would be set if the
wallet was polled in the interval between a block being connected and it
processing the BlockConnected notification.

Thanks to John Newbery <john@johnnewbery.com> for catching this while reviewing
bitcoin#17954.

src/qt/walletmodel.cpp

@@ -82,12 +82,12 @@ void WalletModel::pollBalanceChanged()
         return;
     }
 
-    if(fForceCheckBalanceChanged || m_node.getNumBlocks() != cachedNumBlocks)
+    if(fForceCheckBalanceChanged || numBlocks != cachedNumBlocks)
     {
         fForceCheckBalanceChanged = false;
 
         // Balance and number of transactions might have changed
-        cachedNumBlocks = m_node.getNumBlocks();
+        cachedNumBlocks = numBlocks;
 
         checkBalanceChanged(new_balances);
         if(transactionTableModel)