wallet: Set migrated wallet name only on success

After a wallet is migrated and we are trying to load it, if it could not be
loaded, don't try to set the wallet name.

src/wallet/wallet.cpp

@@ -4306,7 +4306,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
                 success = (wallet != nullptr);
 
                 // When no wallet is set, set the main wallet.
-                if (!res.wallet) {
+                if (success && !res.wallet) {
                     res.wallet_name = wallet->GetName();
                     res.wallet = std::move(wallet);
                 }