> PyOS_string_to_double() is a public function. The check is needed in case 
> 3rd party code uses it without extra checks.

In that case, I'd suggest something more like my modifications to dtoa.c.  By the time that you end up with a negative 'e', all sorts of other things have already gone wrong, possibly including undefined behaviour from signed integer overflow.  IF we need to modify dtoa.c, the check should be earlier IMO.