Make the SVN_FS_FS__LOG_ACCESS option work again.

* subversion/libsvn_fs_fs/cached_data.c
  (dbg_log_access): Adapt to recent API changes. 
  (svn_fs_fs__get_changes): Make sure, we call the debug function
                            with a valid item index.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555227 13f79535-47bb-0310-9956-ffa450edef68