scsi: scsi_debug: num_tgts must be >= 0

Passing the parameter "num_tgts=-1" will start an infinite loop that
exhausts the system memory

Link: https://lore.kernel.org/r/20191115163727.24626-1-mlombard@redhat.comSigned-off-by: Maurizio Lombardi <mlombard@redhat.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -5263,6 +5263,11 @@ static int __init scsi_debug_init(void)
 		return -EINVAL;
 	}
 
+	if (sdebug_num_tgts < 0) {
+		pr_err("num_tgts must be >= 0\n");
+		return -EINVAL;
+	}
+
 	if (sdebug_guard > 1) {
 		pr_err("guard must be 0 or 1\n");
 		return -EINVAL;