Fix linum format string width

[twlz0ne]

`string-width' returns the wrong value:

(string-width "\u2502") ;; => 2
(length       "\u2502") ;; => 1

It causes the following error:

1 Debugger entered--Lisp error: (args-out-of-range 0 6)
2   add-text-properties(0 6 (face linum-highlight-face) #("   1│" 0 5 (face linum)))
3   hlinum-color(linum-highlight-face nil)
4   hlinum-highlight-line()
5   apply(hlinum-highlight-line nil)
6   linum-update-current()

[tom-tan]

Thanks!