This patch fixes 3 classes of bugs in Objects/longobject.c:

(1) declarations of a variable (usually a counter into the digits of
a PyLong) as int instead of Py_ssize_t.
(2) missing (twodigits) casts from multiplies and shifts.
(3) use of '<<' on negative values in _PyLong_AsByteArray.  This
may lead to undefined behaviour, according to the C standards.  (See C99, 
section 6.5.7, paragraph 4).

These bugs haven't manifested themselves in practice.  For (1), there's 
only a problem when dealing with huge integers (more than 2**31 digits).   
The bugs in (2) can only affect platform where the C 'int' type has fewer 
than 32 bits.  (3) could potentially conflict with future compiler 
optimizations, but doesn't seem to be a problem right now.

For these reasons I don't think these fixes should be backported to 2.5.3.

See also issue 4497 for some compiler warnings that need to be silenced.

Fixed in r68890, r68891, r68895 and r68896