Check active even if Condvar times out in PeriodicWorker

[SpriteOvO]

Fixes #60.

Before this PR, we didn't check the active flag if wait_timeout_while returns due to a timeout. There may be a small probability issue with this, i.e. PeriodicWorker being dropped a short time after the wait_timeout_while timeout has returned will potentially result in an infinite join.

[Lancern]

PeriodicWorker being dropped a short time after the wait_timeout_while timeout has returned will potentially result in an infinite join.

The changes LGTM, but is the situation described above really possible?

[SpriteOvO]

but is the situation described above really possible?

Just a conjectural situation, this is mainly to fix the error reported by rustc in #60, and an extra check at least seems harmless. However I can try to write a minimal reproducible later to verify if this situation is possible.

[SpriteOvO]

I was wrong, I can't verify that situation, Condvar::wait_timeout_while always checks for the condition: F, regardless of whether it's woken up or not. It doesn't seem to make much difference whether the lock guard is held or not here. Let's merge it just for suppressing the rustc lint error.