Upstream: Test for NULL vd

It seems we managed to get a deadman triggered during export?

 : 0xffffff8004ebda40 mach_kernel : _return_from_trap + 0xe0
 : 0xffffff7f8942bbbf org.openzfsonosx.zfs : _vdev_deadman + 0x1f
 : 0xffffff7f8941149a org.openzfsonosx.zfs : _spa_deadman + 0xca
 : 0xffffff7f896a6246 org.openzfsonosx.zfs : _taskq_thread + 0x4a6

module/zfs/spa_misc.c

@@ -616,7 +616,7 @@ spa_deadman(void *arg)
 	zfs_dbgmsg("slow spa_sync: started %llu seconds ago, calls %llu",
 	    (gethrtime() - spa->spa_sync_starttime) / NANOSEC,
 	    ++spa->spa_deadman_calls);
-	if (zfs_deadman_enabled)
+	if (zfs_deadman_enabled && spa->spa_root_vdev != NULL)
 		vdev_deadman(spa->spa_root_vdev, FTAG);
 
 	spa->spa_deadman_tqid = taskq_dispatch_delay(system_delay_taskq,