Merge r1439623 from trunk:

Fix error because of negative rate-limit
PR : 52964
Submitted by: Tianyin Xu <tixu cs ucsd edu>
Submitted by: jailletc36
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1455219 13f79535-47bb-0310-9956-ffa450edef68