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