proxy_util: NULL terminate the right buffer in 'send_http_connect'

trunk patch: http://svn.apache.org/r1490994
Submitted by: jailletc36
Reviewed by: covener, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1491466 13f79535-47bb-0310-9956-ffa450edef68