the problem and solution are discussed here
	http://9fans.net/archive/2009/01/234
since the post, i removed the underflow test as i found
it was not effective, at least on the 386.  you may disagree.

further note: there is also lib/ap/gen/strtod.c which
seems to suffer from many overflow/underflow problems.
i didn't change it because i don't understand why another
copy of strtod is necessary.