Eliminate loop in ap_proxy_string_read(). Need to be able to handle

getting an empty brigade from ap_get_brigade(). Also made sure that
we always call ap_get_brigade() in readline mode.

Submitted/Reviewed by: Adam Sussman & Bill Stoddard


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92690 13f79535-47bb-0310-9956-ffa450edef68