#324 win32pipe.popen functions can crash

[Roger Upole]

The popen functions don't check the result of _fdopen,
resulting in a crash after the limit of open CRT
stdio streams is reached.
This limit is 512 by default, but can be raised to 2048 using _setmaxstdio.

Setting to low priority since the built-in python popen functions or subprocess module are usually used in preference to win32pipe.popen* functions.

Python itself also has this same issue, see bug #886488:
http://sourceforge.net/tracker/index.php?func=detail&aid=886488&group_id=5470&atid=105470