Follow-up to r1639319: Fix file object lifetime.

* subversion/libsvn_fs_fs/stats.c
  (read_phys_pack_file): Close rev / pack file only at the end of the
                         pack file.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639436 13f79535-47bb-0310-9956-ffa450edef68