mod_proxy_http: follow up to r1853409: don't play with meta-buckets.

It's better/simpler to handle the final 0-size chunk within the loop (on EOS).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1853561 13f79535-47bb-0310-9956-ffa450edef68