Merge r1572896, r1572911 from trunk:

mod_deflate:
Don't fail when asked to flush inflated data to the user-agent and that
coincides with the end of stream ("Zlib error flushing inflate buffer").
PR 56196.

Submitted By: [Christoph Fausak <christoph.fausak glueckkanja com>]
Committed By: ylavic


mod_deflate: follows up r1572896.
Be safe from successive or post end-of-stream flush buckets.

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1610397 13f79535-47bb-0310-9956-ffa450edef68