hurd: Fix calling vm_deallocate (NULL)

Only call vm_deallocate when we do have the old buffer, and check for
unexpected errors.

Spotted while debugging a msgids/readdir issue on x86_64-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230625231751.404120-3-bugaevc@gmail.com>