bug#12867: 24.3.50; easy-to-repro crash involving mode line

[martin rudalics]

> The need to redisplay the mode line on every move of point is a killer
> of many redisplay optimizations.  It is also potentially expensive by
> itself, because it requires Emacs to count lines, something that is
> not an easy operation in Emacs, which sees the buffer text as a linear
> array of characters, not a series of lines.  So the display engine
> tries very hard to avoid redisplaying the mode line if it decides that
> the line number does not need to appear.

I think it would make sense to use a cache for maintaining line numbers.

martin