fix: datarace in walletmodel.cpp causing occasional segfaults in instantsend code at shutdown (#4583)

Thanks thephez

src/qt/walletmodel.cpp

@@ -74,6 +74,10 @@ void WalletModel::updateStatus()
 
 void WalletModel::pollBalanceChanged()
 {
+    if (node().shutdownRequested()) {
+        return;
+    }
+
     // Try to get balances and return early if locks can't be acquired. This
     // avoids the GUI from getting stuck on periodical polls if the core is
     // holding the locks for a longer time - for example, during a wallet