core: Follow up to r1664205.

Don't let invalid r->proto_num/protocol out of read_request_line() reach
the output filters (when responding with 400 Bad Request).
Suggested by: rpluem


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1664576 13f79535-47bb-0310-9956-ffa450edef68