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.

Objects/unicodeobject.c

@@ -9072,6 +9072,7 @@ _PyUnicode_TransformDecimalAndSpaceToASCII(PyObject *unicode)
             int decimal = Py_UNICODE_TODECIMAL(ch);
             if (decimal < 0) {
                 out[i] = '?';
+                out[i+1] = '\0';
                 _PyUnicode_LENGTH(result) = i + 1;
                 break;
             }
@@ -9079,6 +9080,7 @@ _PyUnicode_TransformDecimalAndSpaceToASCII(PyObject *unicode)
         }
     }
 
+    assert(_PyUnicode_CheckConsistency(result, 1));
     return result;
 }