Harden FSFS f7 index reader against corruption.  The idea is to compare
header information to rev / pack file sizes etc. and error out when
they don't match.  This e.g. prevents OOM due to corrupt table sizes.

* subversion/libsvn_fs_fs/index.c
  (get_l2p_header_body,
   get_p2l_header): Add lots of data checking code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622959 13f79535-47bb-0310-9956-ffa450edef68