Merge r702867 from trunk:

* Recheck again if idle workers are still available when we are signaled that
  they are. This is needed since it can happen that we are signaled by a
  worker thread that went idle but received a context switch before it could
  tell us. If it does signal us later once it is on CPU again there might be
  no idle worker left. See
  https://issues.apache.org/bugzilla/show_bug.cgi?id=45605#c4

PR: 45605
Submitted by: Denis Ustimenko <denusk gmail.com>
Reviewed by: rpluem, jim, gregames


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@705872 13f79535-47bb-0310-9956-ffa450edef68