Possible NULL-related SystemError when interpreter is not fully initialized #161863

Code:

Here IMPORTLIB(interp) can be NULL under rare conditions:

For example, when interpreter is not yet fully initialized.

Output:

» ./Programs/_freeze_module crash crash.py crash.h
Exception ignored in the internal traceback machinery:
SystemError: null argument to internal routine
# ...

Refs #161860 where I first found this case.
Refs #142662 where the initial reproducer was found.

I guess we should raise a different SystemError here with the text that imports are not yet fully initialized.

My proposal is to make the error message better here: it should state the interpreter is not fully initialized.

And ideally we can fix the freeze command as well later.