Fix incorrect size check in NFGenMsg (#287)

Fix incorrect size check in NFGenMsg, resolving TableFamily issue for rules in monitor

monitor_test.go

@@ -156,6 +156,9 @@ func TestMonitor(t *testing.T) {
 		*gotChain.Hooknum != *postrouting.Hooknum {
 		t.Fatal("no want chain", gotChain.Type, gotChain.Name, gotChain.Hooknum)
 	}
+	if gotRule.Table.Family != nat.Family {
+		t.Fatal("rule wrong family", gotRule.Table.Family, gotRule.Table.Name)
+	}
 	if len(gotRule.Exprs) != len(rule.Exprs) {
 		t.Fatal("no want rule")
 	}

util.go

@@ -38,7 +38,7 @@ type NFGenMsg struct {
 }
 
 func (genmsg *NFGenMsg) Decode(b []byte) {
-	if len(b) < 16 {
+	if len(b) < 4 {
 		return
 	}
 	genmsg.NFGenFamily = b[0]