LU-5938 mdd: fixed oops when dereferencing structure

LU-5938 mdd: fixed oops when dereferencing structure

In mdd_changelog_ns_store() and mdd_changelog_data_store(),
lu_ucred(env) can be NULL, so do not dereference it.

Signed-off-by: frank zago <fzago@cray.com>
Change-Id: I45d0cbbb171f05ee1d04e628a3b31c256e0d3951
Reviewed-on: http://review.whamcloud.com/13619
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>