When the target for backupwallet is the source wallet file or directory, the wallet file is cleared to zero length

### Reproduce
bitcoin-cli backupwallet ~/.bitcoin/

### Expected behaviour
Command should fail

### Actual behaviour
Wallet is destroyed.

Reported here
https://www.reddit.com/r/btc/comments/719kz3/just_lost_4bch_backing_up_my_walletdat_file/

Fix proposed in #11376 
{
    if (activeTxn)
        return;
    
    unsigned int nMinutes = 0;
    if (fReadOnly)
        nMinutes = 1;
    env->dbenv->txn_checkpoint(nMinutes ? gArgs.GetArg("-dblogsize", DEFAULT_WALLET_DBLOGSIZE) * 1024 : 0, nMinutes, 0);
}
void CWalletDBWrapper::IncrementUpdateCounter()
{
    ++nUpdateCounter;
}
void CDB::Close()