gh-134768: Fix definition of `mt_continue_should_break()` (#134769)

In 121ed71f4e395948d313249b2ad33e1e21581f8a, mt_continue_should_break
was changed to be guarded by `Py_DEBUG`, but it's used in `compress_mt_continue_lock_held`
with just `assert`, so it needs to be available when `NDEBUG` is undefined
too.

`Py_DEBUG` implies `NDEBUG` is undefined, so we can check just that.

Fixes: 121ed71f4e395948d313249b2ad33e1e21581f8a