Currently scsi_dh_lookup() doesn't check for NULL as a device name. This
combined with nvme over dm-mpath results in the following messages
emitted by device-mapper:

 device-mapper: multipath: Could not failover device 259:67: Handler scsi_dh_(null) error 14.

Let scsi_dh_lookup() fail fast on NULL names.

[mkp: typo fix]

Cc: <stable@vger.kernel.org> # v4.16
Signed-off-by: NJohannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: NHannes Reinecke <hare@suse.com>
Reviewed-by: NBart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>