[fix] Made shardless cluster not cause an assertion error when getting destroyed

[Henonicks]

After cluster.cpp:162 happens, executing cluster.cpp:163 causes the assertion fail. This also works the other way around.

Shutting down the cluster (dpp::cluster::shutdown) before cluster.cpp:162 or the destruction of the object makes the assertion fail go away.

The issue isn't caused by dpp::request_queue::~request_queue directly. This destructor marks queues as terminating which doesn't let new requests come in. Yet after dpp::cluster::tick_timers has already been called so it needs to, well, tick all the timers. The timer queue can end up depleting before tick_timers finishes its job since in the while (true) loop (matching do at timer.cpp:66) we pop the queue and never check if it's empty. And if it's supposed to be unable to, well, that's exactly what caused this issue in the first place. The only place in that loop's function where we do check is above the do, at timer.cpp:62. My guess is that these checks happen to be quick enough when there are no shards to cause that assertion fail before the cluster gets to shut down, though I'm not that familiar with the codebase to be able to assert a confident verdict.

Either way, this PR fixes it by simply checking for the next_timer's emptiness before accessing its .top(). Since the original check virtually instantly exits tick_timers if it succeeds and otherwise accesses the queue anyway, my addition should have no effect on a bot's functionality. Regardless, I tested it with mine for a few minutes and it ran fine.

I decided to maintain the order of the calls in dpp::cluster's destructor in case it was important for graceful exiting.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[netlify]

✅ Deploy Preview for dpp-dev ready!

Name	Link
🔨 Latest commit	e734c1d
🔍 Latest deploy log	https://app.netlify.com/projects/dpp-dev/deploys/6a788f195e391100080c22f0
😎 Deploy Preview	https://deploy-preview-1639--dpp-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.