LU-5369 mdt: check lock handle instead assert

LU-5369 mdt: check lock handle instead assert

The lock handle could be NULL inn some corner case.
We should check it instead of LBUG.

Lustre-change: https://review.whamcloud.com/44905
Lustre-commit: 5e4411e99cd7d0ccf4e51fac1442673844626639

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I1afa7f8c129c104b012ae23141318365c388c503
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46019
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>