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

[martin rudalics]

> FWIW, I recently added such a cache (in Elisp) to nlinum.el, and on my
> test case (largish file with C-v/M-v style scrolling) the difference was
> very significant (but the previous code had no cache at all, not even
> base_line_pos).

So we now already have a syntax-ppss cache and a nlinum cache.  Couldn't
we maintain one cache per buffer, updated by one and the same algorithm?
That algorithm would parse syntax when requested, for example, by
font-locking.  Newlines would be always parsed.  Cache invalidation
would happen as present.

martin