Harden FSFS's revision number parsing.  Replace the combination
of tokenizing and SVN_STR_TO_REV with a strict parsing function.

* subversion/libsvn_fs_fs/low_level.c
  (parse_revnum): New revnum parsing function.  It detects invalid
                  values, overflows and invalid terminators.
  (read_change,
   svn_fs_fs__parse_representation,
   svn_fs_fs__read_noderev,
   svn_fs_fs__read_rep_header): Use the new function to parse revnums.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622931 13f79535-47bb-0310-9956-ffa450edef68