netdump: Check the return value of ifunit_ref()

We may fail to match if the specific interface doesn't exist or was
renamed.

PR:		273715
Reported by:	grembo
MFC after:	1 week

sys/netinet/netdump/netdump_client.c

@@ -451,6 +451,8 @@ netdump_configure(struct diocskerneldump_arg *conf, struct thread *td)
 		CURVNET_SET(vnet0);
 		ifp = ifunit_ref(conf->kda_iface);
 		CURVNET_RESTORE();
+		if (ifp == NULL)
+			return (ENODEV);
 		if (!DEBUGNET_SUPPORTED_NIC(ifp)) {
 			if_rele(ifp);
 			return (ENODEV);