core: Follow up to r1664205 (previously backported)
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
Backports: r1664576



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x-merge-http-strict@1768971 13f79535-47bb-0310-9956-ffa450edef68