Merge r1750218 from trunk:

Follow up to r1737447: fix max_spare_threads lower bound.
Suggested by: Rick Houser <rick.houser jackson.com>

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1750488 13f79535-47bb-0310-9956-ffa450edef68