gh-123913: Fix NULL handling in _curses_initscr_impl of _cursesmodule

[sobolevn]

Thanks to @n-bes for the help with this bug.

• Issue: cursesmodule uses winobj which might be NULL #123913

[picnixz]

I think I addressed it in #123910 (I think we can use #123290 to track the curses bugs unless you want this specific one addressed in a separate issue since it's not entirely related to a reference leak) [my PR is to prepare a refactorization of the curses module]

[sobolevn]

Oh, you beat me by just 2 hours :)
Great work. Closing this PR in favor of #123910

[picnixz]

(Actually, I worked on curses since last week; I just split my PR into multiple ones instead. The original one is #123630).