nflog: Fix Log() crash when gossip is nil (#1064)

nflog/nflog.go

@@ -360,9 +360,11 @@ func (l *nlog) Log(r *pb.Receiver, gkey string, firingAlerts, resolvedAlerts []u
 		},
 		ExpiresAt: now.Add(l.retention),
 	}
-	l.gossip.GossipBroadcast(gossipData{
-		key: e,
-	})
+	if l.gossip != nil {
+		l.gossip.GossipBroadcast(gossipData{
+			key: e,
+		})
+	}
 	l.st[key] = e
 
 	return nil

nflog/nflog_test.go

@@ -264,3 +264,12 @@ func TestGossipDataCoding(t *testing.T) {
 		require.Equal(t, in, out, "decoded data doesn't match encoded data")
 	}
 }
+
+func TestNilGossipDoesNotCrash(t *testing.T) {
+	nl, err := New()
+	if err != nil {
+		require.NoError(t, err, "constructing nflog failed")
+	}
+	err = nl.Log(&pb.Receiver{}, "key", []uint64{}, []uint64{})
+	require.NoError(t, err, "logging notification failed")
+}