Follow-up to r1635528: Fix FSX index rewrite code for empty regions.

* subversion/libsvn_fs_x/index.c
  (svn_fs_x__l2p_index_from_p2l_entries): If we skip an item, the resulting
                                          list will be one item shorter.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1635638 13f79535-47bb-0310-9956-ffa450edef68