Cleanup the bb brigade, because buckets inserted to it can be created from

scpool and this pool can be freed before this brigade.
POSSIBLE (but as yet unconfirmed) fix for crashes seen with threaded servers,
e.g. PR 50335.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1550061 13f79535-47bb-0310-9956-ffa450edef68