bug#59038: loading this base64 file makes emacs -Q 28.2 peg a core infinitely

[Gregory Heytings]

> > That file opens just fine in other modes
>
> It also opens fine in c-mode, with global-font-lock-mode disabled.

Indeed, obviously it's a c-mode font-locking related bug.

> > Note that this bug has nothing to do with long lines.
>
> I imagine that the font-lock issue is related to the line being in 
> excess of 21,000 chars (but general redisplay obviously doesn't have 
> problems with lines this 'small').
>
> Reduced to 10,208 chars that file opens instantly under emacs -Q in 
> c-mode with font-lock enabled; but at 10,209 chars it hangs Emacs (I 
> killed it after waiting 4 minutes).

Interesting, thanks for the bisection!