pythongh-126238: Fix possible null pointer dereference of freevars in _PyCompile_LookupArg (python#126239)

* Replace Py_DECREF by Py_XDECREF

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

@@ -901,7 +901,7 @@ _PyCompile_LookupArg(compiler *c, PyCodeObject *co, PyObject *name)
             c->u->u_metadata.u_name,
             co->co_name,
             freevars);
-        Py_DECREF(freevars);
+        Py_XDECREF(freevars);
         return ERROR;
     }
     return arg;