[scudo][standalone] Lists fix

Summary:
Apparently during the review of D69265, and my flailing around with
git, a somewhat important line disappeared.

On top of that, there was no test exercising that code path, and
while writing the follow up patch I intended to write, some `CHECK`s
were failing.

Re-add the missing line, and add a test that fails without said line.

Reviewers: hctim, morehouse, pcc, cferris

Reviewed By: hctim

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69529