Merge r583817, r583830 from trunk:

* server/util_filter.c (ap_filter_flush): Ensure that the brigade is
  empty before returning.

* server/util_filter.c (ap_filter_flush): Tweak comment; no functional
change.

PR: 36780
Submitted by: jorton
Reviewed by: jorton, minfrin, rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@800333 13f79535-47bb-0310-9956-ffa450edef68