[3.12] gh-106350: Tkinter: do not ignore return value of mp_init() (GH-106351) (GH-107258)

(cherry picked from commit b5ae7c4)

Co-authored-by: Christopher Chavez <chrischavez@gmx.us>

Author: miss-islington

Misc/NEWS.d/next/Library/2023-07-03-03-46-20.gh-issue-106350.LLcTEe.rst

@@ -0,0 +1,2 @@
+Detect possible memory allocation failure in the libtommath function :c:func:`mp_init`
+used by the ``_tkinter`` module.

Modules/_tkinter.c

@@ -876,8 +876,9 @@ asBignumObj(PyObject *value)
         return NULL;
     }
     hexchars += neg + 2; /* skip sign and "0x" */
-    mp_init(&bigValue);
-    if (mp_read_radix(&bigValue, hexchars, 16) != MP_OKAY) {
+    if (mp_init(&bigValue) != MP_OKAY ||
+        mp_read_radix(&bigValue, hexchars, 16) != MP_OKAY)
+    {
         mp_clear(&bigValue);
         Py_DECREF(hexstr);
         PyErr_NoMemory();