scsi: scsi_dh: Don't look for NULL devices handlers by name

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>

drivers/scsi/scsi_dh.c

@@ -112,6 +112,9 @@ static struct scsi_device_handler *scsi_dh_lookup(const char *name)
 {
 	struct scsi_device_handler *dh;
 
+	if (!name || strlen(name) == 0)
+		return NULL;
+
 	dh = __scsi_dh_lookup(name);
 	if (!dh) {
 		request_module("scsi_dh_%s", name);