bug#18419: [Regression] Line number missing when enabling linum-mode

[martin rudalics]

> Steps to reproduce:

The following scenario should be simpler:

1. emacs -Q
2. M-: (setq resize-mini-windows nil) RET
3. Open a file such as src/dispextern.h
4. M-x linum-mode
5. Mouse-drag the mode line up once and then down

My machine is too slow to make such dragging a reasonable exercise but
the gaps show up immediately here.  And for the record: `nlinum-mode'
suffers the same problem.

martin