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 
{
    LOCK(cs_db);
    assert(mapFileUseCount.count(strFile) == 0);
    u_int32_t flags = DB_SALVAGE;
    if (fAggressive)
        flags |= DB_AGGRESSIVE;
    std::stringstream strDump;
    Db db(dbenv, 0);
    int result = db.verify(strFile.c_str(), nullptr, &strDump, flags);
    if (result == DB_VERIFY_BAD) {
        LogPrintf("CDBEnv::Salvage: Database salvage found errors, all data may not be recoverable.\n");
        if (!fAggressive) {
            LogPrintf("CDBEnv::Salvage: Rerun with aggressive mode to ignore errors and continue.\n");
            return false;
        }
    }
    if (result != 0 && result != DB_VERIFY_BAD) {
        LogPrintf("CDBEnv::Salvage: Database salvage failed with result %d.\n", result);
        return false;
    }