Follow-up to r1533816:  We have two nested loops and cleaning the
iterpool in the inner one will destroy the ENTRIES varible of the
outer loop.

* subversion/libsvn_fs_fs/pack.c
  (pack_range): use independent pools for the nested loops

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/log-addressing@1534138 13f79535-47bb-0310-9956-ffa450edef68