GH-96187: Prevent _PyCode_GetExtra to return garbage for negative indexes (GH-96188)

Author: pablogsal

Misc/NEWS.d/next/Core and Builtins/2022-08-22-21-33-28.gh-issue-96187.W_6SRG.rst

@@ -0,0 +1,2 @@
+Fixed a bug that caused ``_PyCode_GetExtra`` to return garbage for negative
+indexes. Patch by Pablo Galindo

Objects/codeobject.c

@@ -1339,7 +1339,7 @@ _PyCode_GetExtra(PyObject *code, Py_ssize_t index, void **extra)
     PyCodeObject *o = (PyCodeObject*) code;
     _PyCodeObjectExtra *co_extra = (_PyCodeObjectExtra*) o->co_extra;
 
-    if (co_extra == NULL || co_extra->ce_size <= index) {
+    if (co_extra == NULL || index < 0 || co_extra->ce_size <= index) {
         *extra = NULL;
         return 0;
     }