Harden FSFS's low-level path parsers against data corruption.

* subversion/libsvn_fs_fs/low_level.c
  (read_change
   svn_fs_fs__read_noderev): All paths must be canonical and start with '/'

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622937 13f79535-47bb-0310-9956-ffa450edef68