Verify result of seek operations in two cases where we didn't.

* subversion/libsvn_ra_serf/commit.c
  (create_put_body): Use svn_io_file_seek.

* subversion/libsvn_ra_serf/update.c
  (create_update_report_body): Use svn_io_file_seek.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1471379 13f79535-47bb-0310-9956-ffa450edef68