Prepare FSX representation parser to handle in-txn data that
is still missing checksums.

* subversion/libsvn_fs_x/low_level.c
  (svn_fs_x__parse_representation): Allow for missing checksums.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568602 13f79535-47bb-0310-9956-ffa450edef68