On the fsfs-format7 branch: Fix the issue of missing items
in the l2p index of some pack files.

* subversion/libsvn_fs_fs/pack.c
  (write_l2p_index): skip empty entries and thus don't use them
   when counting for all entries that need to be written to l2p

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1481446 13f79535-47bb-0310-9956-ffa450edef68