Merge r1632742, r1634836 from trunk:

mod_buffer: Forward flushed input data immediately and avoid (unlikely)
access to freed memory.

Submitted by: ylavic, jailletc36
Reviewed by: covener, rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1673871 13f79535-47bb-0310-9956-ffa450edef68