gh-104536: Improve multiprocessing.process._cleanup logic (#104537)

Fix a race condition in the internal `multiprocessing.process` cleanup
logic that could manifest as an unintended `AttributeError` when calling
`BaseProcess.close()`.

---------

Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/multiprocessing/process.py

@@ -61,7 +61,7 @@ def parent_process():
 def _cleanup():
     # check for processes which have finished
     for p in list(_children):
-        if p._popen.poll() is not None:
+        if (child_popen := p._popen) and child_popen.poll() is not None:
             _children.discard(p)
 
 #