Use-after-free when initscr() or newterm() follows new_prescr() #155875

Bug description:

curses.new_prescr() returns a screen object that calls delscreen() when it is collected. initscr() and newterm() adopt that pending screen but leave it owned by the pre-screen object, so letting that object go out of scope deletes the screen the program is running on.

import curses, pty

def start(fd):
    pre = curses.new_prescr()       # the documented way to set options first
    curses.use_env(False)
    return curses.newterm(None, fd, fd)

screen = start(pty.openpty()[1])    # pre goes out of scope here
screen.stdscr.addstr(0, 0, "x")

$ TERM=xterm-256color ./python repro.py; echo "rc=$?"
Segmentation fault (core dumped)
rc=139

curses.initscr() in place of newterm() crashes the same way.

Expected: adopting the pending screen leaves it owned by whatever the program is now using, so letting the new_prescr() object go out of scope does not free a live screen.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155875: Fix new_prescr screen lifetime #155914