qt: Add missing lock in WalletModel::listCoins

Another problem detected by cs_wallet lock detection (#3401).

src/qt/walletmodel.cpp

@@ -502,6 +502,7 @@ void WalletModel::listCoins(std::map<QString, std::vector<COutput> >& mapCoins)
     std::vector<COutput> vCoins;
     wallet->AvailableCoins(vCoins);
 
+    LOCK(wallet->cs_wallet); // ListLockedCoins, mapWallet
     std::vector<COutPoint> vLockedCoins;
     wallet->ListLockedCoins(vLockedCoins);