The following code leads to XXX Undetected errors in debug builds of
trunk and 3.0:

import _struct
_struct.pack_into(b"8", bytearray(1), None)

Besides that, there's something fishy happening in non-debug builds: 

2.6:
>>> _struct.pack_into(b"8", bytearray(1), None);
>>> _struct.pack_into(b"8", bytearray(1), None);
>>> import sys
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: an integer is required

3.0:
>>> _struct.pack_into(b"8", bytearray(1), None)
>>> _struct.pack_into(b"8", bytearray(1), None)
SystemError: Objects/longobject.c:433: bad argument to internal function


Found with Fusil.

The problem is that, unlike PyInt_AsSsize_t, PyLong_AsSsize_t expects
its argument to already be a long object and else raises the SystemError.

It should probably behave like PyInt_AsSsize_t and raise the TypeError
since in 3.0 it's used in many places where PyInt_AsSsize_t was used.

The attached patches at least correct the XXX undetected error.

Isn't PyNumber_AsSsize_t designed for this purpose?