qt: Add missing LOCKs for locked coin functions

These don't aquire the wallet lock internally, so the caller has to do
it.

src/qt/walletmodel.cpp

@@ -531,20 +531,24 @@ void WalletModel::listCoins(std::map<QString, std::vector<COutput> >& mapCoins)
 
 bool WalletModel::isLockedCoin(uint256 hash, unsigned int n) const
 {
+    LOCK(wallet->cs_wallet);
     return wallet->IsLockedCoin(hash, n);
 }
 
 void WalletModel::lockCoin(COutPoint& output)
 {
+    LOCK(wallet->cs_wallet);
     wallet->LockCoin(output);
 }
 
 void WalletModel::unlockCoin(COutPoint& output)
 {
+    LOCK(wallet->cs_wallet);
     wallet->UnlockCoin(output);
 }
 
 void WalletModel::listLockedCoins(std::vector<COutPoint>& vOutpts)
 {
+    LOCK(wallet->cs_wallet);
     wallet->ListLockedCoins(vOutpts);
 }