[3.12] gh-102509: Start initializing ob_digit of _PyLongValue (GH-102510) (#107464)

gh-102509: Start initializing `ob_digit` of `_PyLongValue` (GH-102510) (cherry picked from commit fc130c4) Co-authored-by: Illia Volochii <illia.volochii@gmail.com>

Author: miss-islington

Misc/NEWS.d/next/Security/2023-03-07-21-46-29.gh-issue-102509.5ouaH_.rst

@@ -0,0 +1,2 @@
+Start initializing ``ob_digit`` during creation of :c:type:`PyLongObject`
+objects. Patch by Illia Volochii.

Objects/longobject.c

@@ -163,6 +163,9 @@ _PyLong_New(Py_ssize_t size)
  }
  _PyLong_SetSignAndDigitCount(result, size != 0, size);
  _PyObject_Init((PyObject*)result, &PyLong_Type);
+ /* The digit has to be initialized explicitly to avoid
+ * use-of-uninitialized-value. */
+ result->long_value.ob_digit[0] = 0;
  return result;
 }