Fix/CI: Handle potential null, add Miri to CI

[lvkv]

The data argument supplied to the cleanup callback we hand to PAM can technically be null. This shouldn't normally happen (given the whole point of the cleanup callback), but the possibility needs to be handled regardless. This was actually caught by running miri on the test update included in this PR:

test module::tests::cleanup_disaster_scenarios ... error: Undefined Behavior: constructing invalid value: encountered 0, but expected something greater or equal to 1:
    |
90  |         unsafe { Unique { pointer: NonNull::new_unchecked(ptr), _marker: PhantomData } }
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0, but expected something greater or equal to 1
    |

Not unrelatedly, this PR also updates CI to add a miri test job.

[Copilot]

Pull request overview

This PR hardens the PAM cleanup callback against a possible null data pointer (preventing UB when constructing Box<T> from a null raw pointer) and adds a CI job to run Miri so similar issues are caught automatically.

Changes:

• Add a defensive null check in cleanup<T> before Box::from_raw.
• Extend/rename the unit test to cover “null data” and panic-on-drop disaster scenarios.
• Add a GitHub Actions Miri job and adjust doc-test comment wording.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
pam/src/module.rs	Adds null guarding in the PAM cleanup callback and updates tests to cover the null case.
.github/workflows/rust.yml	Introduces a Miri CI job to detect UB in tests.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.