bpo-45329: Fix freed memory access in pyexpat.c (GH-28649)

(cherry picked from commit 0742abd)

Co-authored-by: TAGAMI Yukihiro <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Library/2021-10-01-13-09-53.bpo-45329.9iMYaO.rst

@@ -0,0 +1,2 @@
+Fix freed memory access in :class:`pyexpat.xmlparser` when building it with an
+installed expat library <= 2.2.0.

Modules/pyexpat.c

@@ -1204,10 +1204,10 @@ static void
 xmlparse_dealloc(xmlparseobject *self)
 {
     PyObject_GC_UnTrack(self);
+    (void)xmlparse_clear(self);
     if (self->itself != NULL)
         XML_ParserFree(self->itself);
     self->itself = NULL;
-    (void)xmlparse_clear(self);
 
     if (self->handlers != NULL) {
         PyMem_Free(self->handlers);