LU-6142 lov: guard against class_exp2obd() returning NULL.

LU-6142 lov: guard against class_exp2obd() returning NULL.

class_exp2obd() can return NULL.  Sometimes this is tested for,
sometimes not.
Add appropriate tested so that a NULL return is never dereferenced.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ib78514e8e709971ae6d08a11186f77d6c950b1a2
Reviewed-on: https://review.whamcloud.com/39379
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>