bpo-34087: Fix buffer overflow in int(s) and similar functions (GH-8274)

`_PyUnicode_TransformDecimalAndSpaceToASCII()` missed trailing NUL char.
It caused buffer overflow in `_Py_string_to_number_with_underscores()`.

This bug is introduced in 9b6c60cb.
(cherry picked from commit 16dfca4d82)

Co-authored-by: INADA Naoki <methane@users.noreply.github.com>

Python/pystrtod.c

@@ -391,6 +391,8 @@ _Py_string_to_number_with_underscores(
     char *dup, *end;
     PyObject *result;
 
+    assert(s[orig_len] == '\0');
+
     if (strchr(s, '_') == NULL) {
         return innerfunc(s, orig_len, arg);
     }