bug#24751: 26.0.50; Regex stack overflow not detected properly (gets "Variable binding depth exceeds max-specpdl-size")

[npostavs]

>> @@ -869,6 +869,7 @@ main (int argc, char **argv)
>>  
>>        /* Don't let regex.c overflow the stack.  */
>>        re_max_failures = lim < extra ? 0 : min (lim - extra, SIZE_MAX) / 
>> ratio;
>> +      emacs_re_safe_alloca = re_max_failures * min_ratio;
>>      }
>>  #endif /* HAVE_SETRLIMIT and RLIMIT_STACK and not CYGWIN */
>
>   . we shouldn't set re_max_failures to zero if the amount of stack is
>     less than 'extra', since in that case we will allocate the failure
>     stack off the heap;

Then what should we set it to?  Maybe we shouldn't modify it at all,
since the stack isn't actually a limiting factor?