Fix object pool's internal reference counting.

* subversion/libsvn_subr/object_pool.c
  (remove_unused_objects): only decrement the counter if we actually
                           destoryed an entry because ref-counting is
                           not synchronized with hash access
  (object_ref_cleanup): use the actual counters since hash buckets may
                        contain more than one entry
  (insert): new entries count as "unused" until actually handed out 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541307 13f79535-47bb-0310-9956-ffa450edef68