gh-99537: Use Py_CLEAR() function in C code (#99686)

Replace "Py_XDECREF(var); var = NULL;" with "Py_CLEAR(var);".

Don't replace "Py_DECREF(var); var = NULL;" with "Py_CLEAR(var);". It
would add an useless "if (var)" test in code path where var cannot be
NULL.

Modules/_xxsubinterpretersmodule.c

@@ -2383,8 +2383,7 @@ channel_list_interpreters(PyObject *self, PyObject *args, PyObject *kwds)
     goto finally;
 
 except:
-    Py_XDECREF(ids);
-    ids = NULL;
+    Py_CLEAR(ids);
 
 finally:
     return ids;