Create the pool bucket after we know the true length, because

Create the pool bucket after we know the true length, because
(currently) we can't shorten the length of such a bucket by
setting e->length.  (We could split and then remove the extra
bucket, but this seems cleaner.  We may still want to have a
way to shorten a bucket, but there isn't a pressing need for
that at the moment.)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86790 13f79535-47bb-0310-9956-ffa450edef68