On the fsfs-format7 branch: improve robustness

* subversion/libsvn_fs_fs/cached_data.c
  (block_read): add a minor sanity check


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1444691 13f79535-47bb-0310-9956-ffa450edef68