bug#23869: 25.0.95; replace-match can crash emacs

[Stefan Monnier <monnier@iro.umontreal.ca>]

> Morale: a buffer-modification hook should always save-match-data.

Indeed.  If it does modify the match data, it's a bug in that hook function.

> +  /* The functions below modify the buffer, so they could trigger
> +     various modification hooks (see signal_before_change and
> +     signal_after_change), which might clobber the match data we need
> +     to adjust after the replacement.  So we save and restore the
> +     match data around the calls.  */
> +  ptrdiff_t sub_start = search_regs.start[sub];
> +  ptrdiff_t sub_end = search_regs.end[sub];
> +  save_search_regs ();

I think it's important to avoid a crash, but it's not important to
behave correctly even if the change-hooks modifying the match data.
So if we can find a simpler/cheaper way to avoid the crash, it'd
be preferable.


        Stefan