rpc: Prevent dumpwallet from overwriting files

Prevent arbitrary files from being overwritten. There have been reports
that users have overwritten wallet files this way. It may also avoid
other security issues.

Fixes #9934. Adds mention to release notes and adds a test.

test/functional/wallet-dump.py

@@ -7,7 +7,7 @@
 import os
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import assert_equal
+from test_framework.util import (assert_equal, assert_raises_jsonrpc)
 
 
 def read_dump(file_name, addrs, hd_master_addr_old):
@@ -105,5 +105,8 @@ class WalletDumpTest(BitcoinTestFramework):
         assert_equal(found_addr_chg, 90*2 + 50)  # old reserve keys are marked as change now
         assert_equal(found_addr_rsv, 90*2) 
 
+        # Overwriting should fail
+        assert_raises_jsonrpc(-8, "already exists", self.nodes[0].dumpwallet, tmpdir + "/node0/wallet.unencrypted.dump")
+
 if __name__ == '__main__':
     WalletDumpTest().main ()