Fix infinite loop when reading certain data due to two HTTP_IN filters being

present for internally redirected requests.

If HTTP_IN is present, r->proto_input_filters would have it, so adding it
twice is wrong.

PR: 10146
Reviewed by:	Brian Pane


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95895 13f79535-47bb-0310-9956-ffa450edef68