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 
#include "db.h"
#include "addrman.h"
#include "fs.h"
#include "hash.h"
#include "protocol.h"
#include "util.h"
#include "utilstrencodings.h"
#include <stdint.h>
#ifndef WIN32
#include <sys/stat.h>
#endif
#include <boost/thread.hpp>
CDBEnv bitdb;
void CDBEnv::EnvShutdown()