Fix a segfault seen on daedalus earlier this evening. All that was happening

was that we forgot to check whether ctx->bb was empty, so when we pulled the
first bucket, it would be the sentinel, and we'd segfault trying to apr_bucket_read
the sentinel.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91641 13f79535-47bb-0310-9956-ffa450edef68