Merge r1538490 from trunk:

c->sbh can be unexpectedly NULL when the thread that pulls the ready keepalive
connection out of the queue laps the thread that put it on the queue.


Submitted by: covener
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1541207 13f79535-47bb-0310-9956-ffa450edef68