Merge r598299, r599393 from trunk:

mod_filter: don't segfault on (unsupported) chained FilterProviders.
PR 43956


Since we don't support chained filters, and can't expect to while the
filter_init problem remains, we should make it clear to users at startup time.

Submitted by: niq
Reviewed by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@602472 13f79535-47bb-0310-9956-ffa450edef68