txdb: handle malformed first coin cursor key #35191

Fixes #35172.

I am still getting familiar with the codebase, so doing easy tickets for now. Thanks for bearing with me.

What?

CCoinsViewDB::Cursor() caches the first coin key after seeking into the chainstate DB. If the first DB_COIN entry is malformed, CDBIterator::GetKey() returns false, but the cursor warmup path ignored that return value.

Because CoinEntry defaults its key marker to DB_COIN, a malformed key like just C could leave the cursor reporting Valid() == true and GetKey() == true even though the key was not decoded.

This makes the warmup path match CCoinsViewDBCursor::Next(): if the iterator is invalid or the key cannot be decoded, invalidate the cached key.

Scenario

This would require an already-corrupted or manually modified local chainstate LevelDB where the first coin key is readable by LevelDB but not decodable as a CoinEntry.

Normal node operation should not create this key shape, so this is a correctness/hardening fix rather than a common user-facing issue.

Testing

The regression test writes a single-byte C key, which enters the DB_COIN keyspace but is too short to decode as a CoinEntry. It then reopens the DB through CCoinsViewDB and checks that the returned cursor is invalid.

cmake --build build --target test_bitcoin -j8
build/bin/test_bitcoin --run_test=coins_tests_dbbase/malformed_first_coin_key_cursor_invalid
build/bin/test_bitcoin --run_test=coins_tests_dbbase

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utACK c02c4aea2a0c126f9defb255b3da695e2ee619bc

Reviewed the diff. This makes the initial CCoinsViewDB::Cursor() warmup consistent with CCoinsViewDBCursor::Next() by invalidating the cached key when the first DB_COIN key cannot be decoded, instead of exposing the default DB_COIN marker as a valid cursor entry. The regression test covers the malformed first-key case. I did not run the tests.