bug#25391: 24.5; ffap-guesser "stack overflow in regexp matcher" error may crash emacs.

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Thierry Volpiatto <thierry.volpiatto@gmail.com> writes:

> Reproduce from emacs -Q:
>
> 1) require ffap.
> 2) Open a file of about 560k containing only one line, e.g
> "~/emacs.d/elpa/archives/archive-contents"
> 3) M-: (ffap-guesser)
>
> Debugger entered--Lisp error: (error "Variable binding depth exceeds 
> max-specpdl-size")
>   looking-at("^.*\\<\\(Type\\|Name\\|Path\\|Host\\|Port\\) *= *\\(.*\\) *$")
[...]
>
> This is reproductible from emacs-26 as well.

The max-specpdl-size error looks like #24751 (I was planning to push the
patch for that today), but as far as I know, that's only in emacs-26.
emacs-25 and earlier correctly throw "stack overflow in regexp matcher".
Unless you have REL_ALLOC enabled (which is the default for GNU/Linux
distributions with the latest glibc), in which case any buffer search
may crash emacs instead.  That's #24358, occurs in 25.1 (and probably
earlier), but it's already fixed in the emacs-25 branch.  Also fixable
by configuring with REL_ALLOC=no.