gh-99537: Use Py_SETREF() function in C code (#99657)

Fix potential race condition in code patterns:

* Replace "Py_DECREF(var); var = new;" with "Py_SETREF(var, new);"
* Replace "Py_XDECREF(var); var = new;" with "Py_XSETREF(var, new);"
* Replace "Py_CLEAR(var); var = new;" with "Py_XSETREF(var, new);"

Other changes:

* Replace "old = var; var = new; Py_DECREF(var)"
  with "Py_SETREF(var, new);"
* Replace "old = var; var = new; Py_XDECREF(var)"
  with "Py_XSETREF(var, new);"
* And remove the "old" variable.

Objects/setobject.c

@@ -1270,8 +1270,7 @@ set_intersection_multi(PySetObject *so, PyObject *args)
             Py_DECREF(result);
             return NULL;
         }
-        Py_DECREF(result);
-        result = newresult;
+        Py_SETREF(result, newresult);
     }
     return result;
 }