gh-102471: Change PyLongWriter_Discard(NULL) to do nothing

It's convenient to be able to call PyLongWriter_Discard(NULL) in
error handling code.

Author: vstinner

Doc/c-api/long.rst

@@ -824,6 +824,6 @@ The :c:type:`PyLongWriter` API can be used to import an integer.
 
    Discard a :c:type:`PyLongWriter` created by :c:func:`PyLongWriter_Create`.
 
-   *writer* must not be ``NULL``.
+   If *writer* is ``NULL``, no operation is performed.
 
    The writer instance and the *digits* array are invalid after the call.

Objects/longobject.c

@@ -6953,6 +6953,10 @@ PyLongWriter_Create(int negative, Py_ssize_t ndigits, void **digits)
 void
 PyLongWriter_Discard(PyLongWriter *writer)
 {
+    if (writer == NULL) {
+        return;
+    }
+
     PyLongObject *obj = (PyLongObject *)writer;
     assert(Py_REFCNT(obj) == 1);
     Py_DECREF(obj);