bug#53136: 28.0.90; segfault in lock_file

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: 53136@debbugs.gnu.org
> Date: Sun, 09 Jan 2022 19:43:47 +0800
> 
>  - lock_info.dot and lock_info.colon are NULL.
>  - lock_filename is a Lisp string, the data is
>    "/home/oldosfan/Mail/archive/sent/2022-01".
>  - handler, subject_buf are NULL
>  - dot is NULL

Maybe that's the problem: it means current_lock_owner returned early
due to some error, and we are dereferencing a NULL pointer.

Paul, could you please take a look?