ab: follow up to r1811664.

apr_socket_send() can return both an error and data, account for data in
the latter case (i.e. let next call fail, if any).



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1814118 13f79535-47bb-0310-9956-ffa450edef68