zed: Fix zed ASSERT on slot power cycle

We would see zed assert on one of our systems if we powered off a
slot.  Further examination showed it returning a zfs_retire_recv()
with a GUID of 0, which in turn would return a NULL nvlist.  Add
in a check for a NULL nvlist go get around this.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>

cmd/zed/agents/zfs_retire.c

@@ -430,6 +430,11 @@ zfs_retire_recv(fmd_hdl_t *hdl, fmd_event_t *ep, nvlist_t *nvl,
 		    &vdev)) == NULL)
 			return;
 
+		if (vdev == NULL) {
+			fmd_hdl_debug(hdl, "No vdev nvlist for guid %llu\n", vdev_guid);
+			return;
+		}
+
 		devname = zpool_vdev_name(NULL, zhp, vdev, B_FALSE);
 
 		nvlist_lookup_uint64_array(vdev, ZPOOL_CONFIG_VDEV_STATS,