gh-154751: Keep the screen alive in curses.initscr() after newterm()

[fedonman]

curses.initscr() called while a screen from curses.newterm() was current returned a
window built with no screen. The window's screen field is the reference that keeps a
screen alive, so this window kept nothing alive. Dropping the screen freed its WINDOW,
and the next method call on the window read freed memory:

import curses, gc, os
s1 = os.openpty()[1]; s2 = os.openpty()[1]
a = curses.newterm('xterm', s1, s1)
b = curses.newterm('xterm', s2, s2)
curses.set_term(a)
w = curses.initscr()
curses.set_term(b)
del a; gc.collect()
w.addstr(0, 0, 'boom')     # Segmentation fault, rc=139

When the current screen came from newterm(), initscr() now returns that screen's own
standard window, so there is only ever one wrapper over that WINDOW and it holds the
screen. Otherwise it passes state->topscreen, matching newwin(), newpad() and
getwin(); that value is NULL for the screen created by initscr(), which has no screen
object, so plain initscr() use is unchanged.

This restores what the screen objects documentation promises: "Each window keeps its screen
alive, so a screen remains valid as long as any of its windows does."

newterm() and set_term() are new in 3.16 (gh-90092, GH-151748), so no released version
is affected and there is no NEWS entry.

The reproducer above now prints no crash and exits 0, and
curses.initscr() is screen.stdscr is True where it was False. Tests:

$ ./python -m test -u all -v test_curses -m 'test_initscr_after_newterm_keeps_screen_alive'
test_initscr_after_newterm_keeps_screen_alive (test.test_curses.ScreenTests.test_initscr_after_newterm_keeps_screen_alive) ... ok
OK

$ ./python -m test -u all test_curses
Total tests: run=164 skipped=3
Result: SUCCESS

• Issue: Use-after-free when curses.initscr() follows newterm() #154751

[serhiy-storchaka]

Returning the screen's existing window instead would fix that too

And this is right.

Please add a test for this case.

[serhiy-storchaka]

LGTM. 👍