[3.12] Fix possible null pointer dereference of freevars in _PyCompile_LookupArg (gh-126238) (#126474)

[3.12] gh-126238: Fix possible null pointer dereference of freevars in _PyCompile_LookupArg (GH-126239)

* Replace Py_DECREF by Py_XDECREF

(cherry picked from commit 8525c93)

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Peter Bierma <[email protected]>

Author: 3 people

Python/compile.c

@@ -1835,7 +1835,7 @@ compiler_make_closure(struct compiler *c, location loc,
                     c->u->u_metadata.u_name,
                     co->co_name,
                     freevars);
-                Py_DECREF(freevars);
+                Py_XDECREF(freevars);
                 return ERROR;
             }
             ADDOP_I(c, loc, LOAD_CLOSURE, arg);