Prevent out-of-bound array access when the FSFS f7 index information
got corrupted.

* subversion/libsvn_fs_fs/index.c
  (get_l2p_header_body): Verify that the index file actually provided
                         data for the requested revision.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1559870 13f79535-47bb-0310-9956-ffa450edef68