Ensure backupwallet fails when attempting to backup to source file

Previous behaviour was to destroy the wallet (to zero-length)

src/wallet/db.cpp

@@ -672,6 +672,11 @@ bool CWalletDBWrapper::Backup(const std::string& strDest)
                     pathDest /= strFile;
 
                 try {
+                    if (fs::equivalent(pathSrc, pathDest)) {
+                        LogPrintf("cannot backup to wallet source file %s\n", pathDest.string());
+                        return false;
+                    }
+
                     fs::copy_file(pathSrc, pathDest, fs::copy_option::overwrite_if_exists);
                     LogPrintf("copied %s to %s\n", strFile, pathDest.string());
                     return true;

test/functional/walletbackup.py

@@ -190,6 +190,16 @@ class WalletBackupTest(BitcoinTestFramework):
         assert_equal(self.nodes[1].getbalance(), balance1)
         assert_equal(self.nodes[2].getbalance(), balance2)
 
+        # Backup to source wallet file must fail
+        sourcePaths = [
+            tmpdir + "/node0/regtest/wallet.dat",
+            tmpdir + "/node0/./regtest/wallet.dat",
+            tmpdir + "/node0/regtest/",
+            tmpdir + "/node0/regtest"]
+
+        for sourcePath in sourcePaths:
+            assert_raises_rpc_error(-4, "backup failed", self.nodes[0].backupwallet, sourcePath)
+
 
 if __name__ == '__main__':
     WalletBackupTest().main()