Don't segfault if the filter doesn't have any request config prepared.

This can happen when the config doesn't tell us to do anything but the
administrator coded us in Add{Input|Output}Filter.  Instead of segfaulting,
we turn into a noop.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86748 13f79535-47bb-0310-9956-ffa450edef68