Following up on r1557287, resolve the same leak in the other location where we
create temporary buckets to throttle the xml parser. Also avoid using a
cleared pool by allocating the serf allocator in the right pool.

* subversion/libsvn_ra_serf/update.c
  (process_pending): Explicitly destroy bucket after passing it to the inner
    handler.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557687 13f79535-47bb-0310-9956-ffa450edef68