Follow-up to r1555297: Fix a NULL-pointer access.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_file_delta_stream): Check the target's data_rep only
                                      if there is one.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555340 13f79535-47bb-0310-9956-ffa450edef68