sign: skip signing SIGHASH_SINGLE inputs with no corresponding output #35984

SIGHASH_SINGLE only commits to the output at the input's index. If the output at such position doesn't exist, it commits to no output at all (legacy uses a fixed sighash of 1, segwit v0 zeroes hashOutputs), which means the signature stays valid even when outputs are swapped, which is a footgun that lets funds be redirected without the owner's consent.

SignTransaction() already skipped these inputs, but SignPSBTInput() did not, so walletprocesspsbt signed them. This moves the check into the CreateSig so both paths, and any future one, skip producing the detached signature.

Note: if there is a valid use for the segwit v0 case, I would rather re-allow it through an explicit opt-in arg than by default, so it is always a deliberate choice.

Fixes #35977

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35984.

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Thanks for pushing a fix for #35977 so quickly (with extra segwit-v0 coverage, guess you rediscovered it independently). Could you add Fixes [#35977](/bitcoin-bitcoin/35977/) to the PR description so the issue auto-closes on merge?

Thanks for pushing a fix for #35977 so quickly (with extra segwit-v0 coverage, guess you rediscovered it independently). Could you add Fixes #35977 to the PR description so the issue auto-closes on merge?

Funny, I didn't check the open issues, only PRs. Done.

Concept ACK If you think it would help, consider adding a characterization test commit before the fix commit to document the previous behavior and to simplify the fix commit.