Small fix to image width computation

org.el (org-display-inline-image--width): The variable
display-line-numbers-width is nil by default.

lisp/org.el

@@ -16879,7 +16879,7 @@ buffer boundaries with possible narrowing."
                     (/ (or (and (bound-and-true-p visual-fill-column-mode)
                                 (or visual-fill-column-width auto-fill-function))
                            (when auto-fill-function fill-column)
-                           (- (window-text-width) display-line-numbers-width))
+                           (- (window-text-width) (or display-line-numbers-width 0)))
                        (float (window-total-width)))))
         width)))
    ((numberp org-image-actual-width)