fix: segmentation fault that was triggered when initializing a new timer and a reset was called at the same time

[algebraic-dev]

This PR fixes a segmentation fault that was triggered when initializing a new timer and a reset was called at the same time.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase ab606ba7541e65dc1e7ba695a50772de9891f3d0 --onto dd28f005889dd2fcca6fe0638133de561a655ad1. You can force Mathlib CI using the force-mathlib-ci label. (2025-12-05 04:44:46)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase ab606ba7541e65dc1e7ba695a50772de9891f3d0 --onto d3dda9f6d4428a906c096067ecb75e432afc4615. You can force reference manual CI using the force-manual-ci label. (2025-12-05 04:44:47)

[TwoFX]

Is it possible to have a test for this?

[algebraic-dev]

It’s a data race between the next and cancel functions, when the next function needs to return a promise that is inside the timer structure. (outside of the libuv lock) but cancel is setting it to NULL at the same time. I don't think it's easy to test this.