Fix / silence a few more conversion warnings.

* subversion/libsvn_fs_fs/index.c
  (svn_fs_fs__l2p_index_append,
   auto_open_p2l_index): Safe shortening cast.
  (get_p2l_header): Use alternative variable with the same contents.
                    Use maximally widening cast where appropriate.
  (p2l_page_info_copy): Safe shortening cast.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1628415 13f79535-47bb-0310-9956-ffa450edef68