LU-4098 lmv: kernel crash due to misconfigured MDT

LU-4098 lmv: kernel crash due to misconfigured MDT

There are few places with access to lmv->tgts[] without check for NULL.
Usually it may happens when MDT configured starting from index 1
instead of 0. For example:
  mkfs.lustre --reformat --mgs --mdt --index=1 /dev/sdd1

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I7d9bc8876bb0b2c2669050904d4629069b61e639
Reviewed-on: http://review.whamcloud.com/7941
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>