Increase fuzz coverage in the wallet #27272

The wallet has (almost) no coverage: https://maflcko.github.io/b-c-cov/fuzz.coverage/src/wallet/index-sort-l.html. We should change that!

Issues like #27271 should be quite easy for high level fuzz targets to find.

(For people that are interested but unfamiliar with the wallet (like me): I suspect that looking at the unit tests could be helpful for figuring out where to start)

I'm working on increasing fuzz coverage in the wallet. Just to track:

• #27647 - for wallet/fees stuff
• #27585 - add more coverage for coin selection

See #23444

This was picked up as part of #28933, but I couldn't reproduce the regression crash by re-introducing it.

Issues like #27271 should be quite easy for high level fuzz targets to find.

Was #28578 supposed to be a regression test for this? cc @brunoerg

Was #28578 supposed to be a regression test for this? cc @brunoerg

No since the issue is not related to the ScriptPubKeyMan stuff.

The wallet has (almost) no coverage: https://maflcko.github.io/b-c-cov/fuzz.coverage/src/wallet/index-sort-l.html. We should change that!

For reference, the current OSS-Fuzz coverage link: https://storage.googleapis.com/oss-fuzz-coverage/bitcoin-core/reports/20231205/linux/src/bitcoin-core/src/wallet/report.html

@brunoerg is planning on opening a new issue which will keep better track of the progress on wallet fuzzing.