Fully initialize custom_operations job_ops

[MisterDA]

It was missing an initializer for .compare_ext (since 3.12.1), the
default is custom_compare_ext_default; and .fixed_length (since
4.08.0) for which NULL is good.

[raphael-proust]

I don't know enough about this part of the code or the intricacies of the underlying system to meaningfully review this code.

With that in mind, my very generic questions are:

• Do we need to change src/unix/lwt_libev_stubs.c where loop_ops and watcher_ops do use this struct?
• Should we add a test with a custom job to the unix test suite?

[MisterDA]

I've seen the issue when I added warning flags to the C compiler which reported fields missing initializers.
The structs watcher_ops and loop_ops are static, so the fields are implicitly initialized to NULL by the compiler, which is not the case for job_ops as it isn't static (as far as my knowledge of C goes).
The fact that we never had a crash related to garbage values for these fields in job_ops makes me think that they're not used...

Do we need to change src/unix/lwt_libev_stubs.c where loop_ops and watcher_ops do use this struct?

Seems the best to me, I missed them because the compiler didn't report a warning.

Should we add a test with a custom job to the unix test suite?

Testing never hurts!

[MisterDA]

• Do we need to change src/unix/lwt_libev_stubs.c where loop_ops and watcher_ops do use this struct?

Done.

• Should we add a test with a custom job to the unix test suite?

I don't think it's necessary because we're already using all the default functions which raise Failure when called, and the new one do too. I don't know what this test would look like.