LU-8018 lov: ld_target could be NULL

LU-8018 lov: ld_target could be NULL

lov_device::ld_target[ost_idx] could be NULL if the OST target is
not filled in lov_device::ld_lov::lov_tgt_desc[ost_idx] yet.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I1eddb49b3c3518828c531af568b851465ccdffa3
Reviewed-on: http://review.whamcloud.com/21411
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>