util_filter: follow up to r1835640: pending_filter_cleanup() precedence.

Register pending_filter_cleanup() as a normal cleanup (not pre_cleanup) so
that the pending filters are still there on pool cleanup, and f->bb is set
to NULL where needed.

Then is_pending_filter() check is moved where relevant.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1835847 13f79535-47bb-0310-9956-ffa450edef68