_multiprocessing Connection methods don't check if handle is valid or 
not. If you close the socket/pipe, Python may crash on operations, 
especially in poll() on FD_SET(...handle, &rdfs).

Example of crash:
----------------------
import _multiprocessing
obj = _multiprocessing.Connection(755)
obj.close()
obj.poll()
----------------------

Attached patch is a proposition of fix to check handle in all 
Connection methods using the handle.

This issue is not critical anymore because jesse.noller fixed the 
crash in r68768.

Python 2.7a0 (trunk:68873M, Jan 23 2009, 08:46:25)
[GCC 4.1.3 20070929 (prerelease) (Ubuntu 4.1.2-16ubuntu2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import _multiprocessing
>>> obj = _multiprocessing.Connection(755)
>>> obj.close()
>>> obj.poll()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: handle out of range in select()

I would prefer a message "I/O operation on closed socket", but you can 
close this issue.

Note: I proposed a similar patch to improve the error message ("I/O 
operation on closed socket") for the socket module in issue #4853.