LU-15146 mdt: mdt_lvb2reply crash fix

LU-15146 mdt: mdt_lvb2reply crash fix

mdt_lvb2body may crash if res->lr_lvb_data is
not allocated, make it tolerant to not initialized
lvb_data pointer.

HPE-bug-id: LUS-9549
Signed-off-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Change-Id: Ie31cbba9187f9b04b3d1f8d2abc59e0612a44b41
Reviewed-on: https://review.whamcloud.com/45334
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>