(fillrand): The assertion was way too weak, due to

(fillrand): The assertion was way too weak, due to
what must be a typo.  Strengthen it to its intended value.
(dopass): Don't use alloca; it's not worth the aggravation here,
since it's used only to get a page-aligned buffer, and page
alignment doesn't buy us much here.  I'm suspicious that alloca
causes problems on some hosts, due to a recent bug report by Adam
Waltman.