Fix nil pointer dereference in sendHeartbeat

Author: apricotbucket28

gateway/gateway_impl.go

@@ -255,9 +255,14 @@ func (g *gatewayImpl) heartbeat() {
 func (g *gatewayImpl) sendHeartbeat() {
 	g.config.Logger.Debug("sending heartbeat")
 
+	sequence := -1
+	if g.config.LastSequenceReceived != nil {
+		sequence = *g.config.LastSequenceReceived
+	}
+
 	ctx, cancel := context.WithTimeout(context.Background(), g.heartbeatInterval)
 	defer cancel()
-	if err := g.Send(ctx, OpcodeHeartbeat, MessageDataHeartbeat(*g.config.LastSequenceReceived)); err != nil {
+	if err := g.Send(ctx, OpcodeHeartbeat, MessageDataHeartbeat(sequence)); err != nil {
 		if errors.Is(err, discord.ErrShardNotConnected) || errors.Is(err, syscall.EPIPE) {
 			return
 		}

gateway/gateway_messages.go

@@ -2,6 +2,7 @@ package gateway
 
 import (
 	"fmt"
+	"strconv"
 
 	"github.com/disgoorg/json"
 	"github.com/disgoorg/snowflake/v2"
@@ -496,6 +497,13 @@ func (MessageDataUnknown) messageData() {}
 // MessageDataHeartbeat is used to ensure the websocket connection remains open, and disconnect if not.
 type MessageDataHeartbeat int
 
+func (m MessageDataHeartbeat) MarshalJSON() ([]byte, error) {
+	if m == -1 {
+		return json.NullBytes, nil
+	}
+	return []byte(strconv.Itoa(int(m))), nil
+}
+
 func (MessageDataHeartbeat) messageData() {}
 
 // MessageDataIdentify is the data used in IdentifyCommandData