Fix airhorn example.

examples/airhorn/main.go

@@ -102,7 +102,7 @@ func messageCreate(s *discordgo.Session, m *discordgo.MessageCreate) {
 // This function will be called (due to AddHandler above) every time a new
 // guild is joined.
 func guildCreate(s *discordgo.Session, event *discordgo.GuildCreate) {
-	if event.Guild.Unavailable != nil {
+	if event.Guild.Unavailable {
 		return
 	}