Various functions in the 'posix' module that take filename arguments
accept bytearray values for those arguments, and mishandle those objects
in a way that leads to segfaults.

Python 3.1 (r31:73572, Jul 23 2009, 23:41:26)
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.mkdir(bytearray(b'/'))
Segmentation fault

There's at least two seperate problems with the way posixmodule handles
these objects. The first is that the code isn't set up to handle NULL
retvals from PyByteArray_AS_STRING(), which occur for zero-byte
bytearray objects. This causes a NULL-pointer dereference in
PyUnicode_FSConverter() if you pass a zero-length bytearray.

The second issue is that release_bytes() calls bytearray_releasebuffer()
with NULL for the first argument, which directly leads to a NULL-pointer
dereference.

I'm attaching a patch against SVN 77001 which should fix both of these
issues.