`ScanForWalletTransactions()` has the following comment about its return value:

```
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
```

This is not strictly true. If `abortrescan` is called, then `ScanForWalletTransactions()` will return `nullptr`, even though the scan wasn't successful to the tip.

This in turn causes a minor bug in `importmulti`, where the call doesn't return the correct error and warning about potentially not finding transactions if `abortrescan` was called.

I think the correct fix is for `ScanForWalletTransactions()` to inform the caller that the rescan was aborted.
{
    auto chain = interfaces::MakeChain();
    std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(*chain, WalletLocation(), WalletDatabase::CreateDummy());
    wallet->SetWalletFlag(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
    BOOST_CHECK(!wallet->TopUpKeyPool(1000));
    CPubKey pubkey;
    BOOST_CHECK(!wallet->GetKeyFromPool(pubkey, false));
}
BOOST_AUTO_TEST_SUITE_END()