Add more plausibility and consistency checks to the FSFS f7 index readers.

* subversion/libsvn_fs_fs/index.c
  (get_l2p_page): Check that we read exactly as much data as we were
                  supposed to.
  (read_entry): Detect corruption in used bits.
  (get_p2l_page): Read exactly one page - instead of possibly more - and
                  then verify that we consumed exactly as much data as
                  we were expected to.
  (p2l_index_lookup): Although anything beyond the actual pack / rev file
                      is considered an "unused" section of it, the index
                      should never report anything not covered by index
                      pages.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631196 13f79535-47bb-0310-9956-ffa450edef68