Empty out the brigade shared by ap_getline()/ap_get_client_block()

on error exit from ap_getline().  Some other code got upset because
the wrong data was in the brigade.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88552 13f79535-47bb-0310-9956-ffa450edef68