LU-10454 mdd: check return value of lu_ucred()

LU-10454 mdd: check return value of lu_ucred()

In mdd_changelog_data_store_by_fid() part of the function checked
for the return value of lu_ucred(), part of it did not. This lead
to NULL pointer dereferencing.

Signed-off-by: Quentin Bouget <quentin.bouget@cea.fr>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iefe9d10191e499aec94415fb6fe0d5d2064f86f0
Reviewed-on: https://review.whamcloud.com/30707
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>