[Python-3000] Use Py_CLEAR only

[Greg Ewing]

Walter Dörwald wrote:

> The "checking for NULL" scenario looks somewhat like this:
> 
> And the "clearing the reference" scenario should look ike this:

But only if the reference is a local to the function,
and it's used in a disciplined enough way, and the
compiler is smart enough. None of those things are
guaranteed true for all uses of Py_DECREF/Py_XDECREF.

--
Greg