This fixes our sanitizer builds which are currently red on master.
This is related to the issue mentioned in the comment in GuessVerificationProgress:
```
// This function assumes the lock on cs_main is already held (see the
// above comment). This is a temporary check until PR15997 is backported.
// https://github.com/bitcoin/bitcoin/pull/15997
AssertLockHeld(cs_main);
```
However, this PR is currently closed, pending a different approach being reviewed in https://github.com/bitcoin/bitcoin/pull/16426
Since it's unlikely that we'll be pulling in a refactor that large any time soon, we can apply this patch to fix the build in the mean time.