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 (!fDbEnvInit)
        return;
    fDbEnvInit = false;
    int ret = dbenv->close(0);
    if (ret != 0)
        LogPrintf("CDBEnv::EnvShutdown: Error %d shutting down database environment: %s\n", ret, DbEnv::strerror(ret));
    if (!fMockDb)
        DbEnv((u_int32_t)0).remove(strPath.c_str(), 0);
}
void CDBEnv::Reset()
{
    delete dbenv;
    dbenv = new DbEnv(DB_CXX_NO_EXCEPTIONS);
    fDbEnvInit = false;
    fMockDb = false;
}
CDBEnv::CDBEnv() : dbenv(nullptr)