Merge r1635644 from trunk:

mod_proxy_wstunnel: abort backend connection on polling error to avoid
further processing (lingering close, SSL shutdown).

Submitted by: ylavic
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1642856 13f79535-47bb-0310-9956-ffa450edef68