Fix unfortunate NULL in spa_update_dspace

After 1325434, we can in certain circumstances end up calling
spa_update_dspace with vd->vdev_mg NULL, which ends poorly during
vdev removal.

So let's not do that further space adjustment when we can't.

Closes #12380

Signed-off-by: Rich Ercolani <[email protected]>

Author: rincebrain

module/zfs/spa_misc.c

@@ -1853,7 +1853,8 @@ spa_update_dspace(spa_t *spa)
 		spa_config_enter(spa, SCL_VDEV, FTAG, RW_READER);
 		vdev_t *vd =
 		    vdev_lookup_top(spa, spa->spa_vdev_removal->svr_vdev_id);
-		if (vd->vdev_mg->mg_class == spa_normal_class(spa)) {
+		if (vd->vdev_mg && 
+		    vd->vdev_mg->mg_class == spa_normal_class(spa)) {
 			spa->spa_dspace -= spa_deflate(spa) ?
 			    vd->vdev_stat.vs_dspace : vd->vdev_stat.vs_space;
 		}