cxgbe: Ignore doomed virtual interfaces when updating the clip table.

A doomed VI does not have a valid ifnet.

Reported by:	Jithesh Arakkan @ Chelsio
Reviewed by:	np
MFC after:	1 week
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D29662

sys/dev/cxgbe/t4_clip.c

@@ -215,6 +215,9 @@ update_clip_table(struct adapter *sc)
 	last_vnet = (uintptr_t)(-1);
 	for_each_port(sc, i)
 	for_each_vi(sc->port[i], j, vi) {
+		if (IS_DOOMED(vi))
+			continue;
+
 		if (last_vnet == (uintptr_t)vi->ifp->if_vnet)
 			continue;