bug#30834: 27.0.50; posn-at-point and display-line-numbers-mode

[Sébastien Chapuis]

Hi,

When display-line-numbers-mode is on and the (point) is on the
beginning of line, posn-at-point returns a wrong value.

Reproducible in emacs -Q:

```
(save-excursion
  (message "Without display-line-numbers:")
  (display-line-numbers-mode -1)
  (goto-char (line-beginning-position))
  (message "column 0: %s" (car (nth 2 (posn-at-point))))
  (forward-char)
  (message "column 1: %s" (car (nth 2 (posn-at-point))))
  (message "With display-line-numbers:")
  (display-line-numbers-mode)
  (goto-char (line-beginning-position))
  (message "column 0: %s" (car (nth 2 (posn-at-point))))
  (forward-char)
  (message "column 1: %s" (car (nth 2 (posn-at-point))))
  )
```

Output:
```
Without display-line-numbers:
column 0: 0
column 1: 19
With display-line-numbers:
column 0: 0
column 1: 114
```

On the bol, it returns zero but it should return (- 114 19)


In GNU Emacs 27.0.50 (build 6, x86_64-pc-linux-gnu, GTK+ Version 3.22.26)
 of 2018-03-13 built on XPS13
Repository revision: ae80854e9a9f8f6733870b5b5a88467867dceb6c
Windowing system distributor 'Fedora Project', version 11.0.11906000
System Description: Fedora 27 (Twenty Seven)

Configured using:
 'configure --prefix=/opt/emacs-git --with-modules --with-xwidgets
 --with-imagemagick CC=clang'

--
Sebastien Chapuis