wallet: reject invalid cached xpubs at load #36638

A wallet file whose descriptor cache holds an invalid extended public key aborts the process at load, in an assertion in CPubKey::Derive.

Unserializing such a key leaves it empty. The cache loaders stored the empty key. A key is invalid if its public key is not a valid point, or if it has depth 0 and a non-zero child number or fingerprint. I return DBErrors::CORRUPT for such a record in both caches, as for a record of the wrong size. I tested this with the extended unit test, which fails without the change and passes with it. A modified wallet file aborted bitcoin-wallet before and reports a corrupt wallet now.

This is one of several PRs from a large audit of Bitcoin Core that I did with the help of LLM tools. The audit produced its evidence all at once, which is why the PRs arrive together. I understand each change fully and will answer review questions myself.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36638
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36638.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36638#author-ralph

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LLM Linter (✨ experimental)

Possible places where named args for integral literals may be used (e.g. func(x, /*named_arg=*/0) in C++, and func(x, named_arg=0) in Python):

• src/wallet/test/walletload_tests.cpp: WalletDescriptor(descriptor, 0, 0, 0, 0)

^{2026-10-24 05:59:49}