staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4098
Reviewed-on: http://review.whamcloud.com/7941Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>