bug#51171: 27.2; `forward-word' w/arg bug in narrowed buffer

[Eli Zaretskii]

> From: Ian Nehera <delta29@shaw.ca>
> Date: Tue, 12 Oct 2021 17:17:14 -0700
> 
> To reproduce from 'emacs -Q /path/to/dcl.c’ (file attached):
>    1. M-x `subword-mode' (side question: should C-c C-w still use the
>       obsolete alias `c-subword-mode'?).
>    2. Navigate to the function 'int dirdcl(void)' and use
>       `narrow-to-defun' (C-x n d).
>    3. From the top of the buffer, use `forward-word' with an arg greater
>       than 1 (like C-u 2 M-f).
>    4. Repeat this with C-x z z z z... until you get an error around the
>       start of the while loop (2nd to last message in list below).
>    5. Moving point forward then trying the same command again will yield
>        the last message with a number greater than `point-max' (5741).
> 
> Other notes:
>       - The bug dissapears with a widened buffer or when narrowing to
>         earlier functions.
>       - `backward-word’ doesn’t seem to have the same problem.
>       - I'm reporting this from OSX 10.14.6 (homebrew), but it also occurs on
>         Arch Linux (emacs being installed from the standard pacman repos).

Stefan, any suggestions wrt this?