Fix UpdatePathAggregator4ByteAs() ignores 32bit value

[floatingstatic]

I noticed that the UpdatePathAggregator4ByteAs() function is returning an error related to malformed attribute list error:

Reason="notification-sent code 3(update) subcode 1(malformed attribute list)" State=BGP_FSM_ESTABLISHED Topic=Peer

A path that triggered this looked something like:

&{0 [] 138 [{MpReach(ipv6-unicast): {Nexthop: 2001:de8:1:2::33, NLRIs: [2404:b5c0::/32]}} {Origin: i} 134911 {Med: 98} {LocalPref: 55} {Aggregate: {AS: 134911, Address: 10.0.0.1}} {Extcomms: [2:2]} {As4Aggregator: {AS: 134911, Address: 10.0.0.1}} {LargeCommunity: [ 65102:1:1]}] []

We can see in the above the Aggregate.AS was 134911 which falls into 32-bit space, but we were only setting the aggregator in cases where the kind was a uint16 here:

gobgp/internal/pkg/table/message.go

Lines 240 to 243 in 0cc8a98

	if attr.Value.Askind == reflect.Uint16 {
	aggAttr = attr
	aggAttr.Value.Askind = reflect.Uint32
	}

And as a result we end up matching this condition and sending a notification:

gobgp/internal/pkg/table/message.go

Lines 253 to 255 in 0cc8a98

	if aggAttr == nil && agg4Attr != nil {
	return bgp.NewMessageError(bgp.BGP_ERROR_UPDATE_MESSAGE_ERROR, bgp.BGP_ERROR_SUB_MALFORMED_ATTRIBUTE_LIST, nil, "AS4 AGGREGATOR attribute exists, but AGGREGATOR doesn't")
	}

This change corrects this to validate the aggregator in cases where aggregator is a 32bit value.

Additionally as I was stepping through the FSM logic I noticed that table.UpdatePathAttrs4ByteAs() returns an error but we do not check the value of the error. So additionally I am adding a small fix to correct that.

[fujita]

Thanks a lot!