Sorry, Stefan Behnel, I still don't get it. alloc will always bigger than size after the if else case:
 
    if (size < alloc / 2) {
        /* Major downsize; resize down to exact size. */
        alloc = size + 1;
    }
    else if (size < alloc) {
        /* Within allocated size; quick exit */
        return 0;
    }
    else if (size <= alloc * 1.125) {
        /* Moderate upsize; overallocate similar to list_resize() */
        alloc = size + (size >> 3) + (size < 9 ? 3 : 6);
    }
    else {
        /* Major upsize; resize up to exact size */
        alloc = size + 1;
    }

Since we limit the alloc at:

    if (alloc > PY_SIZE_MAX / sizeof(Py_UCS4))
        goto overflow;

whenever size > PY_SIZE_MAX / sizeof(Py_UCS4) at first will cause alloc overflow. So why not limit size to PY_SIZE_MAX / sizeof(Py_UCS4) at the beginning?