wallet: Reset chain notifications handler if AttachChain fails

AttachChain will create the chain notifications handler which contains a
reference to the wallet's shared_ptr. If AttachChain fails, the wallet
needs to be unloaded, and this is expected to happen with its custom
deleter ReleaseWallet. However, if the chain notifications handler is
still set, then the shared_ptr is still referenced by something, so the
wallet is never actually released.

Author: achow101

src/wallet/wallet.cpp

@@ -3099,6 +3099,7 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
     if (time_first_key) walletInstance->MaybeUpdateBirthTime(*time_first_key);
 
     if (chain && !AttachChain(walletInstance, *chain, rescan_required, error, warnings)) {
+        walletInstance->m_chain_notifications_handler.reset(); // Reset this pointer so that the wallet will actually be unloaded
         return nullptr;
     }