fix: dont allow bot.start on a started cluster (#1384)

Author: braindigitalis

src/dpp/cluster.cpp

@@ -209,6 +209,10 @@ void cluster::add_reconnect(uint32_t shard_id) {
 
 void cluster::start(start_type return_after) {
 
+	if (start_time != 0) {
+		throw dpp::logic_exception("Cluster already started");
+	}
+
 	auto event_loop = [this]() -> void {
 		auto reconnect_monitor = numshards != NO_SHARDS ? start_timer([this](auto t) {
 			time_t now = time(nullptr);