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 
{
    
    
    
    
    
    
    
    int64_t now = GetTime();
    newFilename = strprintf("%s.%d.bak", filename, now);
    int result = bitdb.dbenv->dbrename(nullptr, filename.c_str(), nullptr,
                                       newFilename.c_str(), DB_AUTO_COMMIT);
    if (result == 0)
        LogPrintf("Renamed %s to %s\n", filename, newFilename);
    else
    {
        LogPrintf("Failed to rename %s to %s\n", filename, newFilename);
        return false;
    }
    std::vector<CDBEnv::KeyValPair> salvagedData;
    bool fSuccess = bitdb.Salvage(newFilename, true, salvagedData);
    if (salvagedData.empty())
    {
        LogPrintf("Salvage(aggressive) found no records in %s.\n", newFilename);
        return false;
    }
    LogPrintf("Salvage(aggressive) found %u records\n", salvagedData.size());
    std::unique_ptr<Db> pdbCopy(new Db(bitdb.dbenv, 0));
    int ret = pdbCopy->open(nullptr,               
                            filename.c_str(),